bring bags/purses into historic areas

You're right, there is no 'one size fits all'. You should check the official website of each site you wish to visit to get current correct information.

There is no standard. So permit small bags but Borghese absolute no bags. Some prohibit cameras or keep them out of sight. At one time the bags for the Colliseum were inspected but allowed. That was a couple of years ago so it could have changed. I would go with smallest bag possible where ever you go. You don't have to keep it straight, they will tell you when you show up at the front door.

The Pacsafe Metro comes in several sizes. Which do you have? I just bought a citysafe CS50 (small but will hold what I need it to) and a CS100. The latter meets the measurements for Pompeii. I think it will be fine in most other places as its within posted measurements. The Borguese Galleria has a bag check area. Why do you need a bag big enough for your camera? The only time my DSL goes in a bag is if its raining. Otherwise, I wear it cross body using a neoprene strap. Super comfortable and easily accessible when needed.

Just went to the Colosseum on Saturday. Taking in my medium sized shoulder bag was no problem.

There was never a total bag ban at the Colosseum but they were banning large bags and backpacks (without really classifying what is 'large') for awhile. They've eased that restriction now but it's still in place for the Forum and Palatine so as advised, smaller is better. If it's not really bulky, I think you'll be fine everywhere but the Borghese where everyone has to check practically everything. You are allowed cameras there now, though.

I usually carry a small to medium fabric type cross body. Enough room for sunglasses, wallet, kleenex and those types of things. I have one that is fairly light weight. After wearing this bags for two or three weeks it can start hurting my neck, so I try to keep it light. I bought it at a sports/travel store because they are made with a little more security. Baggallini is really a great brand, developed by flight attendants. They are made well have lots of pockets and very secure.

At a number of sites, I've seen a backpack (even a small one) banned while a larger shoulder bag was ok.

The thing is that situations can change at any time for all sorts of reasons. Security was heightened at some heavily-visited attractions in Rome and other cities after the Hebdo attacks in Paris last winter. Add a rash of vandalism and theft of structural pieces of the Colosseum, and all of this probably contributed to the concern about largish-bags.

Attractions may also abruptly close due to security threats - as the Eiffel did twice during our last trip to Paris - so no matter how well prepared we are, sometimes we just have to roll with a few unexpected punches!

We used to use a daypack when out and about and we found that if my husband carried it, it got banned, if I carried it over one shoulder and under my arm like a purse, it didn't.

Now we use a messenger bag and take turns but I always carry it when there are doubts about entry with bags.