We are travelling from Florence to Nice via San Gimignano (where we
will be based) for 3 nights. After sight seeing in San Gimignano we
thought of travelling to Nice via Pisa in one day. The distance is
about 300kms. Is this too far for one day's leisurely trip

This is something for you to decide, wouldn't it, depending on: 
- Your means of transportation (don't trust public transportation in Italy)
- Whether or not you are travelling with children or have any other
specific conditions that would require stops;
- 300km = about 4 hours (if you caculate traffic jams or finding parking in Pisa). 

So, leaving at 7AM, you'd arrive there just before noon. You'll have
several hours to tour the city - if all goes well - before you'll have
to drive back (not being too tired, say leaving at 5 or 6 PM).

It's okay for a day trip but in summer the European roads are slowed
with lots of road works.  Allow plenty of time.  Don't forget that
there are heaps of things to see along the way too.  Better to stay
over a few nights if you can.  If you can't, plan your day and factor
in the grid-locked autostradas!