Just how big can GTKRadiant maps be? Bigger/smaller than Halo maps?
Specifically, I'm thinking of BIG outdoor areas like the Pelanor fields in LOTR; just some big plane type thingy.
They can always be but it depends on how you do your terrain, models, other things in the map, AND especially your VIS work can play a big factor to FPS. But it can still be that big. You just have to utalise the engine right.
Things you can do: Use Detail / Structural brushes (Medium Skill) Hinting (Very Advanced Skill) Bigger Terrain brushes (fairly easy) - it may not look as good but it helps the FPS. Less detail on buildings.