Ideal size for travel sling bag

A slling bag or a shoulder bag/messenger bag worn cross body?

I mentioned something about a bag the size of a Vogue magazine - not the September issue...Cross body strap is a good idea, and it should zip.

You will need a map with all the streets marked on it, plus a Metro map, maybe a Bus map. You can buy all this in one booklet at newsstands in Paris for around 6 EU "Paris par arrondissement - L'indispensable" Make sure to get the blue book. Very convenient and you won't get lost.
Whatever else you think you need is optional.
If you want water, carry a small bottle. Snacks should be small, or can be bought at supermarkets.

You don't have to stuff sweaters or rain jackets in your bag - better to carry them. Umbrellas are virtually useless, since there are often windy conditions.

And definitely do not carry wallets with all your cards and cash in them.
Use only what you'll need for the day.

Use a combination wallet that has a place for currency as well as a zippered pocket for coins.

Keep the combination wallet in a zippered compartment in your bag, and know where it is at all times.
Ditto your phone or any other gadgets you might be using.

This is my favorite bag. Purchased one for my husband as well. Husband has orange, I have blue. It's not big. I got it to carry our passports, phone, money, credit card, because I simply couldn't carry 4 passports in a undercover money belt like I use to when I was young and single. We also visited a lot of places notorious for pickpockets. When I got on crowded subways I just rotated it around to the front of my body without having to take it off.

http://www.pacsafe.com/venturesafe-150-gii-cross-body-pack-4.html

Note this is a Pacsafe bag with a lot of anti-theft features. So you may not like going in and out of it all the time. But we felt was a trade-off and got use to use it.

This is very much a personal matter. I carry a full-size purse when I travel. I typically leave my hotel in the morning and don't return until evening. So my bag contains: wallet (smallest I can find), passport (on travel days), sun glasses, water bottle, guidebook pages, local map, smartphone, local travel brochures for the day's activities (sometimes), pills needed during the day, a dose or two of various meds I might need (for headache, etc.), hand lotion, hairbrush, plastic fork/spoon combo, a few facial tissues, a bit of toilet paper (in some countries), a small notebook and sometimes an umbrella. I like having enough extra room for my windbreaker on days that start out cool and then warm up, but I'd give that up if I had to.

I work at reducing the bulk, but I've had to keep adding medical items as I've gotten older. I imagine most people end up carrying more when traveling than they do at home, but there's no reason not to strip your key ring and wallet down to just the items you will need away from home.

My basic list for cities and towns (hiking and beach would be different)

-camera. Mine is a DSLR and takes up 99% the room I have.

-phone (has guidebook and map, otherwise I'd want those too)

• mini power bank and 3 inch cord to charge phone

-wallet with minimal stuff (leave business cards, checks, local loyalty cards, and random junk at home and use a very small wallet)

-baggie with small personal items like chapstick, a pill box with Advil, travel roll of tums, a band aid, tissues

-sunglasses

-scarf tied to the outside if I'm going in churches and need a cover up or if temps are weird

(Other people will have a water bottle -my husband carries ours, a guidebook - I do ebooks, umbrella - I carry/wear a hooded jacket on rain days)

That's it for me and I fit it in a small purse
I use this one but it's nothing special - and crossbody bag with zippers will work

http://www.target.com/p/women-s-crossbody-faux-leather-handbag-with-double-zipper-detail-merona/-/A-15179125

I use a crossbody messenger bag when travelling as in addition to my usual (pared down--some makeup, motrin, sunglasses) purse items I want to carry a guidebook, my little travel notebook, map, granola bar, maybe a bottle of water, my camera and I can even fit a thin sweater in and an umbrella. I leave my regular wallet at home and instead use a small Coach wristlet that is fastened to the lining of my purse at the bottom, yet I can retrieve it and access due to the strap.

You might be making a mountain out of a molehill...

Here's a basic idea of what you might think about purchasing. Inexpensive, has interior pockets, a zip and a flap, and a long adjustable strap. It's really all you'll need:
www.amazon.com/Phoenix-Craft-Leather-Crossbody-Shoulder/dp/B01BLBX02Q/ref=sr_1_112?s=apparel&ie=UTF8&qid=1488481901&sr=1-112&nodeID=3421075011&refinements=p_n_style_browse-bin%3A3388486011

I'd also get one of those fabric tote bags that fold up to the size of your fist in the first supermarket you go to, cost: 1 EU. If you need them to carry anything that isn't valuable, they are very useful.

Frankly, I'd concentrate on having good shoes that you have broken in well, and are prepared to walk 7 or 8 miles a day in.
Feet - not accessories - are the most important, in my opinion.

Very important point for me....the bag itself has to be lightweight. Toting a day's worth of supplies can weigh more than anticipated, so I would not use a leather bag that starts out with significant weight when empty. (I like Baggallini products.)

I just use a simple tote bag, that I got for free for attending a conference.

What's in it:
Umbrella
Guide book
Phrase book and dictionary (if I'm in a country where I know enough of the language to make use of these)
Map
Water/soda (I'm addicted to the latter)
Extra phone battery (mine is one of the few that still has a removable battery; if it didn't, I'd carry a spare power source like this Anker: https://www.amazon.com/gp/product/B01CU1EC6Y).
Empty ziplock bags (very handy to have for all sorts of things)
Lactaid (I need this for milk products)
Wipes (I like the kind in individual packets; I find the pop up box gets messed up in a tote bag).

And various things I may accumulate during the day - small purchases, receipts I want to keep, etc.

None of this is heavy, and except for the umbrella, none of it is large in any one dimension, so this stuff would fit in almost any bag you don't mind carrying around for the day.

I agree with Lynn on the bag weight. I do not take leather or vinyl. I prefer something like nylon. As it happens, my current bag is also a Baggallini. I've used LeSportSac in the past, but I also troll the aisles at Walmart and Target looking for suitable cheap bags. Warning: I found out the hard way that a really cheap black fabric bag will not remain black through a summer-long trip.

Adding that my priorities are:
cross-body strap
zippered main compartment as well as at least 1-2 zipper pockets
Fabric that will be ok if it gets wet
Not too heavy

And most importantly:

fits what I need it to and not much more. Some sites have bag restrictions and might not let you in with a very big bag, plus it just gets tiring carrying around a huge item. When my kids were little I carried a big purse and it worked for my needs then (extra snacks, diapers, toys...). Glad to be done with that, though. Go as small as reasonable for your needs.

I second the idea of a squishable totebag - you want one that folds/squishes smaller than a tennis ball, and unfolds into a grocery bag size. I carry my purse in one on the plane for a little extra room. Then the tote can be used for purchases (some stores don't give bags) or other uses.

I have developed back problems, so need to travel light with my day bag - what seems light in the morning can seem very heavy to my back by late afternoon. My current bag is a quilted Baggallini convertible bag, I think called the Explorer, and I think they don't make it any more. I can switch it between shoulder, cross-body and (if I'm in a rural place with not many people) a backpack. I used to carry a larger bag, same concept, made by Eagle Creek, which also isn't made any more.

I like the versatility of a convertible bag. My current bag has dimensions, I think, of less than 8 x 11. I can hook an umbrella onto it, if necessary, and perhaps fit a small water bottle inside. Other than that, it holds guidebooks and maps, and a small wallet in a secure inside pocket.