Permitted backpack sizes in Pompeii

Pompeii has a baggage storage service onsite.
Other specific historic sites you can check online the rules for what is allowed beforehand

There has never been a single report here of any problem with the Pompeii luggage storage counter.

If it is smaller than her (i.e. it's not sticking out sideways or above her head when she's wearing it), it will be fine in Pompei. What they don't want is someone with a trekker's backpack (i.e. large one with a frame). That said, there are some museums that don't allow backpacks of any size, but they will have storage available (but it may not be a locker).

Thanks for the info. Per the regulations page: "Large backpacks, umbrellas, luggage or bags (cm 30x30x15) and other bulky items are not allowed and must be left at the cloakroom." Maybe it's my reading comprehension, but I'm not sure of the 30x30x15 regulations is for "luggage and bags" or if those are the dimensions by which they define "large backpack."

I used an umbrella while visiting Pompei. It was raining and saw many users. Maybe rules aren’t always enforced?

Diane, well, it is Campania after all, so I guess I'll roll with the idea that rules are more like loose frameworks, to put it mildly (I lived in a suburb of Naples for 8 months, so I mean that with all the anarchic love I have for the region).

The bag doesn't sound large
My questions would be:
1) that may be the max dimensions but it may not be full (if just used as day-trip bag) - great
2) is it heavy? - then check it

Why carry a bag?
Food: The restaurant on-site was so busy, we didn't buy food there
Camera
Raincoat / umbrella
guidebook

We were there April/May 2019
The luggage-check at the main entrance (may have) / has a couple hundred lockers. The computer interface instructions to get a locker door to open was a bit confusing. Keep your receipt - it has the code to get the locker open when you return.

I used this packable backpack in Pompeii without problems. It expands to 18”x10x9, slightly smaller than your daughter’s backpack. It held my daily needs, souvenir book and was easier on my neck than my purse.

Not saying it will work but I've carried a smaller, not-full backpack over one shoulder and tucked under my arm and it flew in "no backpack" sites in Germany and Austria... Is it a backpack if it's not on your back?