DMH can be played on a 2 x 2 board, although we always find it a bit more cinematic with a 3 x 3 one. For number of buildings, as Harry says, 6 plus plenty of fences and some scatter to add cove and that Old West flavour.
Heres a 3 x 3 DMH set up we played a few years ago (it was also used for a 7TV game, too)
Alternatively, let the scenery provide cover, as in this slightly adapted DMH game for 4 players.
RMZ