make them as long as you can! my philosophy is to map for as long as i can until i get bored, then i wrap up whatever i have done. ^_^

it's not the best way to do it, but it works for me! 

The game engine will pretty much limit how big your map can be but knowing the smallest appropriate size is more subjective. With the exception of skill-setting, start or transitional maps, IMHO, E1M1 from Quake would be at the smaller end of a 'full size' single player map.

I think that most speed maps or 100 brush contest maps would be small snack size maps and not have enough meat to be considered full size.

I would also rather play a smaller well made map then a huge one lacking detail or quality.