I have used Global Mapper quite a bit to generate contours for import to SU. The largest I have done was about 2000 acres. You may want to play with the contour interval to speed up SU. I generated a terrain model with 2' contours on 2000 ac. SU ran all night. When It was complete, I had to zoom WAY back (really far). Good Luck, sounds like a cool project.
D