What size purse to bring to Paris, Venice, Munich?

Any bag/purse policies today may be different tomorrow so I suggest smaller than currently required.

My favorite bag is roughly 13" long by 6" wide by 9" tall (measurements are tough as it's lightweight and squishy) and I haven't have to check it anywhere unless checking ALL bags was mandatory.

I can only speak to Paris - as I'm headed there next week and I've done a little looking around. No big bags - no backpacks. Here's what the Louvre says: It is strongly recommanded not to bring suitcases and luggages to the museum. Only luggages not exceeding 55 cm x 35 cm x 20 cm will be accepted. Please accept our apologies for the inconvenience and delay which may be caused by the extra security checks at the entrance.

And just FYI, that was the warning they were using BEFORE last week, i.e. the post-November 2015 warning.

That's 21x13.7.5 approx. I'm going much smaller for my daybag to make life easier. Mine is 11x8x6. I needed to fit the dimensions of a "purse" for RyanAir.

The size Valerie states is perfect: 11 x 8 x 6, with a cross body strap. You can get a water bottle, guide book, iPad, money etc into that size. To give you an example of size, a Longchamp Le Pliage is about as large as I'd go and those are 10 x 10 x 5.

The other sizes Valerie mentions refers to the maximum size you can leave at the coat check, not what can be taken into galleries because that's the size of a roll aboard suitcase.

Backpack purses are almost never allowed in museums to protect statues.

Finally, your purse will always be inspected for security, no matter where you go-- if that's what you meant by bag checks--or did you mean having to leave your purse at the coat check?

Yes, Bets' clarification is helpful. The Musee d'Orsay website says this:
Suitcases, travel bags and backpacks are not permitted in the museum, and cannot be left in the cloakroom.

Centre Pompidou says this: The following objects must be left in the cloakroom before entering the museum or the exhibitions:
suitcases, backpacks, umbrellas, baby carriers, helmets and luggage (maximum: length 35 cm/width 15 cm/depth 30 cm).
That's pretty small - 13.7x11.8x6.

I'm not sure if "luggage" means purse, since suitcase is already listed and backpack is already listed. But what I take from all of it is that small is better - esp. right now.

I just finished a trip to Germany, and carried my Tom Bihn Small Cafe Bag, which is 10.6" tall, 8" wide, and 3" thick. I was not asked to check it in any of the museums I visited. I didn't actually carry books in it, but it handled two smartphones and a number of other things I needed for the day. It would have fit an iPad Mini as well.

Thanks for all the helpful replies! I think I'll go with a little smaller, something more like 11x8x6". It sounds like I'll be able to carry the bag in most places (Yes, by bag check I mean I would like to avoid checking my bag at coat check or in a locker). If anyone else knows of any other rules or has any tips, it's super helpful!

I should add that, at one of the museums, I saw a woman with a tiny backpack, about the size of my bag, being told she had to check it, while I was let through. It probably isn't an issue for @Jess, but backpacks of any size can be problematic.

On the other hand, the museum I was at was particularly small and cramped. I can see why they didn't want people to wear backpacks.

I use a LeSportSac crossbody bag, the kind with only 1 zipper across the top and not two. It's the perfect size for me. My sister uses a small Ameribag - the teardrop-shaped ergonomic brand. In Paris we went to the Louvre, Pompidou, Cluny Museum, and the Musee d'Orsay and were never asked to check our purses.

I went on Rick's Best of Europe in 14 days last June/July which visited each of these cities and more. I took a Travelon Convertible Cross-Body purse everywhere I went and had no problems taking it anywhere. It was just the right size to carry my travel books, maps, a small umbrella, small souvenirs, tiny binoculars, sunglasses...even a folded wind breaker. The dimensions were 12.5 x 11.5 x 3.5 inches. Here is the link to the one I purchased from Amazon.com:
http://www.amazon.com/Travelon-Anti-Theft-Convertible-Crossbody-Power/dp/B00OMGUGV6?ie=UTF8&psc=1&redirect=true&ref_=oh_aui_search_detailpage

Have fun!

Suzette