the only real sizing issue is top tube length...most bikes have about a 14" chainstay, and the seat tubes are pretty much all the same length: too short to make any really noticible difference unless you've been riding long enough to tell, and even then...
basically, i'd reccommend that you get a bike with a 20.5" top tube or longer. the longer ones are 20.65", and 21".
wheel sizes...hmm... basically, there's 20" and 24". if you want to do tricks and such, get a 20" a 24" might fit you better, but they can be more difficult to manuever, and not a lot of people ride street or ramps on them...they're more for dirt jumping and racing...mostly racing.
the specialized should be just fine for you now, as long as it has 3 piece cranks.
or if you've got the cash you could always get a really nice frame (standard, T-1, kink, FBM, Fit) for around $300 and get all the parts separately, and get exactly what you want. but if you're just starting out, i'd say go for a reasonably priced complete bike with 3 piece cranks and start ridin'!