I would also say two days for Venice. I would also note that it's very very easy to find bad food there so collect recommendations and have backup plans. That's probably good advice for all of the cities, actually-- we found our best dinners in Montalcino and in a small town outside of Verona. My favorite was far and away Rome, but I'm very much a big city girl. I'd say three days-- we took a private tour with a private driver and had an overview of the highlights in a day (Spanish Steps, Piazza Navona, Colosseum, Circus Maximus, Pantheon, Forum, Trevi Fountain-- am I missing anything? and the Vatican in the afternoon-- I don't recommend this as exhausted doesn't even begin to cover how tired I was at the end of the day), but three would enable you to see things at a more sane pace.
I was doing ok with the heights on the Leaning Tower until the time came to go up to the tippy tippy top. My height phobic mom made a couple comments that got the fear into my head, and then the combination of the marble steps, plus a pathetic little guard rail which, given the step I would have had to walk around on to get to the ladder, was essentially two feet high and wouldn't hold a sliding person if I slipped, yeah, I couldn't quite do that.
Have a fantastic trip! Italy is a great place!