forgot to add, around 3-5" is ideal, the larger ones dont adjust well to captivity and the smaller ones are "ok", but right around 3-5" is best. probably a 3-4" one. that way you can watch it change colors and it will adjust well.
Oh and the larger ones are more "prone" to nip, but still its a chance as i am sure you know.
There are also the exceptions to the rule, so maybe a larger one will adjust and not eat your SPS.... but then again it might.
thats why i like to try my angels out first with SPS, then throw them into the display.
shoot me a PM if you want to know where to get great angel fish that have been quaratined and can be put in with SPS or might already be in with SPS.....
Nick