Max Map Size?
What is the largest amount of surface area aloud in a Portal map? Ive had trouble with maps that were around 15000x15000, but ive seen GMOD maps on Youtube that look like 50000x50000! How big can you make your map?
Open hammer, make a brush the size of the entire grid, then look at the dimensions.
I kinda know that... 
I mean, I made a huge maps once (read first post again) and it didnt load.
I'm not sure if there is a newer number (and if there is I'd assume it is bigger) but hammer/source can handle 8192 brushes.
You can't make a map fit the ENTIRE dimension of the grid. Somewhere around 16384 units from the edge on all sides, the game starts to render things weird and it has problems. I think there is better documentation about this over at Interlopers, but I know it does happen.
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The maps you are talking about that look like they are extremely large have 3D Skyboxes. If you walk out, you will eventually hit an invisible wall.
See this for more information about 3D-skyboxes, what they are, and how to create one.
Note they only give the feeling the map is bigger. You cannot actually acces them, and they are not part of the gameplay - only of the enviroment.
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