What size compressor best...

I think you need at least a 30 gallon one.

Honestly, you might want to look at just getting an electric die grinder and a stator (I think that's what they're called) so you can control the speed. I did 95% of my head with an air die grinder but it took forever because I had to keep waiting for the compressor to catch up, although I don't actually know how big the one I was using is. I bought one of those cheap 99 cent (plus $15 shipping ) electric die grinders off ebay, and I used it to do my intakes and an exhaust manifold, along with finishing up a couple things on the head. The only real drawback with them is that the collet is not actually 1/4" and some of the long burrs and such will wobble at certain speeds, but really overall it's been great for the money.

I have a 50 gal I use for alot of things and with my die grinders ive ported heads with that and so have some of my friends with it. Even have done a few quick spray jobs on some hoods. For compressors bigger is defanatly better. It suck to have to wait for it to build up pressure agian so you can continue your work. Well any ways good luck.

Go with a makita die-grinder, like stated, no way you want to wait for that air compressor to keep up with you. (believe me, i have two )