There's no rule saying that a P3 casing must be small. There are loads of P2s and P3s out there which run on full towers.

How many watts needed depends on what you plan to build. For a normal budget PC, a 350w Acbel E2 Power or a Fortron Saga 350/400w would be sufficient. Even a coolermaster 380w extreme power should work fine.

Btw, this should belong in Q&A. Can someone please move it.