Ridecamp: [RC] Slant Stall Dimensions

Is there a rule of thumb for how long and wide a stall in a slant load should be? I have a 6 by 12 stock trailer and want to put 2 slant stalls in the back and a storage area in the front. The horses are arabs, no big warmbloods or drafts. Also, is reverse slant better? It does put the last horse's head very close to the back of the trailer if I got hit - opinions here? 

                              Shelly in DE