One small and one medium is considered the standard practice. I got a pair of potter's by doing this. I purchased my two individuals at 1.5" about two months apart. The first one I got had grown about a half inch by the time I got my second one. After six weeks of quarantine, I placed the new "female" into an isolation chamber made of lighting diffuser. It took almost two weeks for the "male" to stop attacking the chamber trying to get her. At that point, I opened the chamber and left it in for about another two weeks, during this time, he would attack her anytime she left the chamber's vicinity. One morning I noticed she was no longer sleeping in isolation, and had found a place in the rocks to sleep. I finally got to remove the isolation chamber.

For the next few months he kept her "captive" in his territory. As long as she was in his area, she was good. If she left, even if only to follow him, he would quickly chase her back. Three weeks ago, I started noticing different behavior in the evenings.

That's when I saw my first spawning runs between the pair (I'm still trying to get it on video). Now that they are truly a male female pair, there is almost no aggression between the two, just an occassional tail nip. I have the whole process fairly well documented, maybe I'll make a Potter's Pair "Build" thread.
So I guess the point of all this, just be prepared for it to take a few months before the two fish will actually pair up and be happy together. It's well worth it though, and now I really want to pair up my flagfin angel. :beer: