taco wrote:
You probably just need to use hint brushes, area portals, func_detail, etc if you are having lag issues. There are a number of goos optimization guides floating around.

But there's no goo in my map! :p

How big a map should be depends on the map really. It should be long enough for the player to be emerged into the map before it ends, but short enough that it doesn't outstay it's welcome.