The lightest one I have is 4Front piano, though you might need to adjust the sounds through EQs, because it has no controls (sad).
For piano, I use the Yamaha preset in Cubase 5, or sometimes, the presets in Reason (ReWired in FL Studio). But for me, the Cubase piano is the best, so I suggest you install Cubase (it's got a wide range of awesome and pro-sounding instruments).
I simply export the piano to an audio file for mixing in FL.
This is probably because all of the piano VST's I've tried in the past are:
1. Disk Space-consuming
2. Not free
3. Heavy on CPU