Just incase the Shoutbox rolls past Seventh's answer...
CT Spawn points are defined by info_player_starts.
T Spawn points are defined by info_player_dethmatch.
And Buyzones are func_buyzones and as a brush entity they are as big as you make them. You can have more than one, actually it is a good idea to have one at each Spawn point that covers all the spawn points in that area.
Most people use armoury entities as top up areas, as the buyzone is disabled by it's timer after game start, but that depends on the settings.