I like to use 10" & 12" round plates at all distances for shooting with .223 and greater. I think they're just a good standard size and are realistically sized targets to be shooting (I.E., they roughly cover the vitals area if you put them up against someones chest). Plus, if Its too easy I can just move the plates out further.... 200, 300, 500yrds...
I've seen a lot of people debate on if you should get 10" or 12" plates. I say buy what you can get and which you'd rather carry out to those nasty distances (They are kind of heavy).
Also, If you'll be shooting them with .223 then I would recommend spending the extra money for AR550. The plates should last just about forever if used ONLY at or beyond 100yrs (closer is also unsafe IMHO). Sub 100yrds you'll start to cause craters with .223 (IMPE, .308 tends to be fine at 100yrds).