whats the biggest rear tire can i fit on my fz750?

for the fz 750 you can fit a 130/80V18. This doesn't give the bike great handling and a common mod is to fit a FZR 1000 (2GH) EXUP swing arm and wheel which will allow you to fit a wider 160 mm tyre and improve things greatly.

Fitting is straight forward enough as it's a like for like bolt up.

Check out FZ750.net for a more info