Hiya, most games (especially snes games) only have very small maps due to the limitations of the machines RAM - the reason they appear so big is that the game world is made up of a big grid of maps, and when you step off the top of one map, it'll put you at the bottom of a new map - which goes someway to hide how small the individual maps are, and thats something you can do in gigas.
the reason i stuck to 100x100 for the maximum, is anything bigger would require a noticeable loading time (100x100 maps can load instantly), and also if i allowed to big a map - people would probably get lazy and try to fit a whole game onto one map.
something to bear in mind is that every frame it does the AI/collision detection/animation for every tile and entity on that map, so if you had 1000 odd entitys on a map, even if a couple were on-screen it'd run far to slow to be playable.
i felt 100x100 was being generous, its friggin huge!