Size of small backpack allowed at Paris sites

We had to check our standard size daypacks at all museums but I was able to take in a small, purse like day pack. My smallest pack is a Jansport Half-pint.. it's large enough for a sweater, kleenex and small water bottle.

The Catacombs is the most strict of these for obvious reasons; unless the backpack is truly a small purse size you may not be allowed in. They have had plenty of experience with people carting off souvenirs.

Generally if you have doubts, have the woman carry the pack over one shoulder; odds are greater it will be allowed with her than a guy. My husband and I learned this years ago when he would be required to check a bag that I was allowed to carry as a purse. Most places will not allow you to carry a backpack on your back in art museums for obvious reasons.

There is no checking facility at the Catacombs and you exit far from where you went in so don't try to bring a large bag there.

I was just at the Eiffel Tower last week. I had a good-sized camera backpack with a DSLR body, three lenses, computer and accessories (http://store.lowepro.com/fastpack-bp-250-aw-ii). It wasn't a problem bringing it up at all. They just wanted to look inside it first.

You could bring the size of about the September issue of Vogue magazine, for example.
I think you don't have to worry about your "significant other" carrying your purse or bag.

The Catacombes have more guards to prevent "losses" inside and at the exits. You might not have been there in some time, janet. The whole entrance is being re-vamped, and more is to come.

Rather than take a chance with any size backpack, why not pick up a cheap messenger style bag which will be allowed in many places where a backpack will not?

Take your backpack and just switch bags while in Paris if going elsewhere on the same trip (that is what I did when hauling camera stuff around)

I would consider trying to take as little as possible on museum days or when you go to visit the monuments. It slows up Security.
You can go back to the hotel and get what you need afterwards.
Bottles of water are available anywhere, so are snacks, etc. A map is foldable.

What is a messenger style bag?

What is a messenger style bag?

http://www.tenba.com/products/messenger-dna-8-graphite.aspx

This is the bag I plan to take to London and Paris for my new Fuji X-E3.

my husband and I travel with messenger bags as our airline carry ons for electronics, meds, cameras etc and then when we are out and about if we feel the need to carry a bag, we use one and take turns carrying it. For around Paris I usually just wear clothing we adequate secure pockets for what is necessary. Women get used to lugging way more weight around in a bag than is needed. I used to get a head ache from the shoulder bag until I stopped carrying a purse most of the time. Scotevest.com and Magellan.com and other travel stores carry lots of shirts, jackets, coats etc with ample hidden pockets for the basics and it works for us most of the time. On day trips where we want to have more things we carry one of the messenger bags.

When I traveled the Catacombs a few years ago there were no guards visible at all below and no place to stow bags, no restrooms etc. Given the odd sudden popularity of the place I can imagine they are doing some renovation if only to add restrooms as it is not reasonable to expect people to wait in line for 2 or 3 hours and spend a couple hours under ground and not have them available. They were when we went pretty strict about not allowing large bags.