I am trying to add areaportals to my map by associating them to doors (those standard, circular doors in Portal 2).

No matter what I do, I get leaks.

My main question is what dimension do I make the func_areaportal brushes. Do I make them the same size as the door instance (which did not work)? Do I make them the same dimension as the func_detail areas (did not work either)? Even if I cover up the area larger than the func_detail dimension, I still get leaks.

I searched the forum, but I did not find any answers. Google did not tell me anything useful either. Most of the examples involve HL2 or CSS mapping, and they have rectangular doors.

I wonder func_areaportal is even possible with these circular doors. Do I have to make the func_areaportal brushes circular, like the door model? And to what depth? I experimented quite a bit, but all my attempts ended up in failure (i.e. leaks).

Because with the door instances the actual door frame is a func_detail, if you create an areaportal that isn't as big as the hallway in which the door resides you'll get an areaportal leak. I made some instances for myself and placed the areaportal so that it was the same dimensions as the frame--192 wide by 128 high, I think--and thin enough so that it didn't stick out either side of the door model.

Your func_areaportal should expand the entire area between the two walls on either side of the door and the floor/ceiling above and below your door. Essentially, you're creating two separate "miniature" maps that cannot touch or visibly see each other without world brushes unless there's an area portal at the hole (in this case, it's the door). Don't make the area portal a circle, leave it a rectangle and make sure it's covering that whole space.

Hope that helps.