what to wear in Duomos - Florence/Pisa

Shoulders and upper arms covered (t-shirt sleeve length is fine), knees covered (so no shorts, capris are fine).

In June 2001 we expected that it would be as strict as The Vatican. But people with shorts were allowed in. I don't know about nowadays though.

Quokka has given correct information and you may be turned away if you try to push the envelope. Some Duomos are also very strict about bags, banning small backpacks.

THanks - I have a sundress that is comes just to my knees and (covering maybe the top half of them) and was concerned that I would be turned away at the Vatican and at other churches in Italy

The main problem with your sun dress is probably the bare arms rather than the length. Your upper arms need to be covered--you could carry something to put over the dress.
My Eyewitness Guide to Rome summarizes "both sexes should cover torsos, upper arms and legs."

I was just there 2 weeks ago. My aunt wore capri shorts and a tshirt (shoulders covered). I wore a knee length skirt with a tank top and a sheer short sleeved top over it. If you climb the dome, I recommend doing this because it was very hot by the time we got to the top (and it was early morning) so I could take off the sheer top when reaching the top of the dome, then put it back on before going back inside (many other women were doing this too). Many also had tank topsbut covered their shoulders with shalls, which is a good option if it is too hot. My aunt wore open toe sandals and I wore sketchers. We wore the exact same things in the Vatican and it was just fine.