I just racked into a secondary, only because I wanted to free up my primary. Do I need to top up my carboy with water?
Definitely not, your batch size is the batch size it should be
How long does the beer stay good for once bottled?
Months if you keep it in a cool place. Better to get it in the fridge after a few weeks bottle conditioning.
How long does a batch of star San last for?
If you keep it in a sealed container, also can be good for a couple months