I made it pretty big.. about as big as 40 nexuses I think.. it still isn't killing them though..
Here is the map:
http://tinyurl.com/yfldomIt's not done yet though so don't bash it please
ADDITION:
Hmm I found one way to kinda get it to work, but it's a really really bad work around..
When player casts dark swarm, location is centered on it, and the nexuses start being given to that player, until he brings one to that location, and then it gives them back to player 11, until player only has 1 nexus left, and then kills that one and removes the dark swarm.
I don't think i can give units back to player 11 though..