Any compact universe should have strings that wind around, just like when you wind a string around a torus.
Well these winding strings, like a rubber band, want to compress any manifold down to the plank length. (At which point the dual momentum strings we discussed dually “push back” to prevent further collapse.) However, winding strings are oriented. Wrapping 3 times clockwise is different then 3 times counter-clockwise.
If we could have winding strings come in contact with anti-winding strings we would have the winding modes unwrap, and the universe expands. However, this process must be in equilibrium to keep the universe big for any length of time.
Well, a string sweeps out a 2 dimensional surface as it moves, so has a 2d interaction area. Same with a anti-winding string. So, the largest dimension where you could maintain long term expansion is a 2+2=3+1 dimensional space which would expand leaving the rest of the dimensions compactified.
This does something more, since it may take a long time before this equilibrium comes about, this could explain the high entropy in the universe.