What do I wear in church?

Shoulders and knees must be covered-both women and men
Sandals are fine

Just your shoulders. No short shorts. To the knee seemed to be acceptable. Men need to take their hats off. Please be respectful if a service is underway.

Everybody must cover Knees and shoulders, the landlord couldn't care less about arms and feet. You are supposed to follow the rule also in Churches that can't afford to pay guards at the doors.

Franciscan monks enter any church hooded, wearing leather sandals and long skirts, but we are not allowed to be as weird as them.

Low-cut tops and bare midriffs aren't acceptable, at least at the Vatican.

Also watch for signs near the entry way related to hats. I've encountered a few churches (not necessarily in Italy) that wanted female visitors as well as males to remove their hats. That's easily done if you know it's the local preference.

There's also a no-sandals rule for the Vatican that's mentioned on some tour-company web sites. (I haven't found a dress code on the Vatican's own web site.) But there are tons of summer visitors to Italy, as well as residents, in sandals, and I've never seen anyone denied entry for wearing them.

In Catholic churches women are supposed to keep their hats on, but men must uncover their heads. That was how I was taught it.
In fact for me this "taking of your hat when indoors" is so deeply ingrained in my upbringing that I will always take my hat of whenever I go inside somewhere, even when getting on a train...

I think it is worth mentioning that there are no hard and fast published rules. These are places of worship, and like your own church you might attend, you only wear a certain standard of clothing when you go to church. Knees and shoulders covered is good guidance, but where there are monitors, they look and decide what seems acceptable. So they don't like midriffs bared, they call it out, another monitor may not care at all. Dress somewhat normally, shoulders and knees covered, something you would feel good wearing to your own church, and you will be fine. This applies whether anyone is standing at the door to check or not.

Welcome to the forum!

I appreciate you asking. Do what you feel would be respectful. However, if you will be visiting in the summer, especially with temperatures like the current heat wave, I think standard shorts would be fine. Personally, if I was attending Sunday Mass I might wear a nicer outfit.

Things have certainly changed since the first time I was in Italy on a high school trip back in 1966. We had to bring a sports jacket and wear a coat and tie to dinner every night!

I hope you have a great trip!