14 days Trip - Pisa - Siena - Florence - Venice ?

I think you should get a guide book to read what each of these cities has to offer. I know some people who are happy with just a few hours in Pisa and others who can spend a few days there. It really depends on what you want to see and do.

As to how to divide your time, that's another place a good guide book can help. Make a list of sites for each city and base your length of stay off that list. You may need 6 days in Florence or you may only need 3 days, it's impossible to say without knowing what you plan to visit.

I would immediately take the train to Venice upon landing and work your way back to Pisa. Book train tickets up to 120 days in advance in order to get the best prices.

Donna

Pisa has some good points but I wouldn't stay there. See the Duomo, Tower, Baptistry and Campo Santo. that takes a few hours at most. Then off to somewhere else. If you want to stay in the general area of Pisa take a look at Lucca or Montecatini Terme, both are smaller and easy to get to Florence on the train, or, go to Florence and day trip to them, either way, four nights (3 days) would do well, maybe one more if you don't want to be rushed.

Siena is easy from Florence also, either a day trip or go and spend a couple of days (3 nights).

I would go from Florence to Venice on the high speed train and spend 3 nights there minimum. The high speed back to Pisa for your outbound.

I would skip Bologna and Mantua this trip, too big a hassle.