You first need to determine what you want the mesa for. Just to give your table a southwestern look? Or will you want to game on it?
Mesas are big, tall mountains/hills that are flat on top. Usually either inaccessible, or accessible in very limited places (look up the Acoma Pueblo).
Given their height, you may wish to build a framework out of cheap styrofoam, then cover it with paper mache. If you're seeking a mesa you can play on, its hard to make one that won't dominate a table.