Will this size camera bag be allowed in museums, etc.?

Yes, should be no problem! Musee d'Orsay has people check backpacks, but all other shoulder bags seemed to be allowed. Just be prepared for security checks, both x-ray (at the Louvre and Eiffel Tower) and visual checks inside the bag (everywhere else).

That's a 17x8 inch bag and I think that's pretty large. When I was there in April, I downsized my normal backpack to an 8x6 thing to make security screening easier. Whether or not he's let in with that will, in the end, be decided at each site. Eiffel Tower says that they can't be too large, which is vague. d'Orsay says no to suitcases, travel bags and backpacks. Orangerie says no to suitcases and luggage. I guess it depends what the camera bag looks like to them.

I just looked up the size of my tote bag: it is 18"w x 12 h x 6"d. I carried it everywhere last month, including the Louvre, d'Orsay, Sacre Coeur, and Eiffel Tower and had no problem--no one blinked. I needed a bag to hold my large camera/water bottles/kids stuff so I couldn't have gone much smaller. I saw many other tourists with backpacks and bags and didn't see anyone turned away, hassled, or pulled out of line--except, as I said, d'Orsay was strict about making everyone check backpacks at the coat check. My husband, sister, and brother-in-law all carried backpacks some days (despite my warnings--like you, I was concerned they'd be turned away) and it was fine.

Mine was a standard women's canvas tote and probably looked nonthreatening from a security profiling perspective. I can't promise they won't judge the camera bag differently if it's black and technical-looking, but size-wise it's comparable to mine. I think it's true that nothing is guaranteed, but in my experience they were relaxed about bag size, since everyone has to go through other security checks.

When nervous about this have the woman carry the bag on entry; we have often been asked to check the messenger bag when my husband was carrying it and never been asked when I was. (when we carry a bag on a day trip, we take one of our large messenger bags and take turns carrying it) It reads as a purse when a woman has it apparently and more like luggage when a man does.

Your bag will be fine. It will be examined closely, but you'll be allowed in - even if a female isn't carrying it.