This is kind of a longshot but...
The size doesn't really matters as long as you don't want to place stuff (such as trees, buildings, rivers, and so on...). If you only want to use it as a warm lighting indirect effect, and won't need to include brushes etc, just make it 256x256 units for example.
I'm attaching the test I made. Open it and use it. You'll see how simple it is.
useful tools and stuff here on TWP 
[spoiler]
[/spoiler]
YAEY it works!
hehe.. finally we can put a rest to this thread 
Thanks everyone for the help!
btw josepezdj, the brushes you use to surround doors are cut WAY cleaner than mine, I might steal those from you
hehe.. finally we can put a rest to this thread
Thanks everyone for the help!
btw josepezdj, the brushes you use to surround doors are cut WAY cleaner than mine, I might steal those from you
Awesome 
looking good
Thanks everyone for the help!
btw josepezdj, the brushes you use to surround doors are cut WAY cleaner than mine, I might steal those from you
Wow man! THAT does look really good... you see? [i]"...with a little help from my friends..."/i] you can do anything!
I'm glad I helped you. I will keep an eye on your release!
You can use those prefabs for the door_frames; they're not mine though, but Damagepy's (he kindly sent them to me in my very first mapping stage!). Thank him!
useful tools and stuff here on TWP [spoiler]
[/spoiler]