Ok, generally pistols are going to fall into the following sort of catagories:
1) Long slide - So these are full size frames and 6" barrels. Typically these are competition guns or something of the sort. Typical example would be something like a CZ Tactical Sport or a Glock 34.
2) Full Size - This is your standard size of weapon. 1911 with a 5" barrel, Glock 17, CZ 75, Beretta 92, etc.
3) Compact - The classic for this catagory is the Glock 19. Similar size would be a Kahr K9, the FNP-9, CZ P-07, Sig P229 or the Springfield EMP. Typicall 4" barrels, still full grip.
4) Subcompact - So, this starts to get a little sketchier, but typically you are looking at something like a Walther PPS, an XD subcompact or the Kel-Tec PF-09. Now you are looking at 3" barrels and short grips.
5) Pocket - So, these are the new category of ultracompact, typically 2", single stack and very short grips. Will fit in your pocket. The Ruger LCP, the Keltec P-11 or P3AT, the Beretta Nano, Kahr CW380.
Obviously these aren't hard and fast rules, because every company is trying to get an edge somewhere, but more or less this is how the market is broken out right now.
Thinkingblade