I am from Nebraska and I know that mulies are basically just in the western parts of the state. I have friends who rance out near Lexington and North Platte and they always see mulies, but tell me that they are seeing fewer and fewer. (The reason they say is that the white tails are overrunning them and pushing them out). I would say your best bet would be in that area, but stay away from the Platte river because that is the white tail haven. Good luck and maybe try an area near McCook?
As far as size goes with western mulies, the typical buck is the normal symmetrical 4x4 with a good amount of spread. Good luck and I hope you're successful. Kepp us updated and dont forget that you can buy your tag online starting Monday the 20th!!