The old addage, bigger is usually better. So the biggest you can find really.

Also the old Ford 5.0 throttle bodies bolt right up from what I have heard. You can source one from a junk yard for about $25-$50 easy. Only thing is you have to fab the wiring from a 4 wire TPS to a 3 wire I beleive. Or its the other way around. I cant remember. I think that TB is like 65mm also.