First trip Italy

You could fill months in any of those places, so I would not sweat it too much. There are always options for day trips. The only way to really allot your days is to make a list of what you want to see and do and then make sure you have given yourself ample time. I think most people give Venice too short a stay to get past the crowds and appreciate it.

I would go with 6 nights in Rome.
5 nights in Florence with day trip to Lucca and/or Sienna.

4 nights in Venice is plenty

What I suggest you do is go on TripAdvisor and put in the name of the city you are visiting and add Things to Do. It will show you many of the key things to see. You may not wish to do everything that is shown, but plan your days around the key items, such as Rome- The Vatican and Sistine Chapel are a must. If you spend any time in the Vatican Museum, you will spend most of your day there. Of course, after leaving, you could stop and see Hadrian's Mausoleum on the Tiber that is referred to as Castel St. Angelo.
Also, you should see the Coliseum as well as the nearby Palatine Hill which has much to see as well as the Roman Forum. There is much more to see in Rome, but that is a start.

The big cities of Italy are fabulous, full of great sights. The smaller towns are magic! I would keep your allocation the way it is (or perhaps give 1 night from Venice to Rome for sightseeing or Florence for day trips) and follow your own suggestions for a day trip from each city.

I agree …. allow yourself the time to wander and get a little lost!

Or you could use the "Explore Europe" link above on this website for the sights our host recommends seeing. I think your time allocation is about right, given the total time you have.

Our first trip to Italy was 17 nights.
We did 3 Venice, 3 Florence then 2 Siena, 2 Montepulcinao, 2 Assisi
ended with 5 in Rome.

We really enjoyed the small towns especially in the mornings and evenings.
I would recommend trying to stay a few nights NOT in a big city

Maybe 3 Venice, 2 or 3 Florence
then pick a hill town or 2 for 2 nights each and end with 5 in Rome

Thank you all so much, this is helpful and makes me feel better. I believe you when you say you could spend a month in each place, so much to do an see.