When we would haul 3 horses in a 3 slant, we would put the smallest in the first stall because it seemed smaller than the last two. It worked good and we never had any problems. But eventually the smaller horse which was mine, seemed to get claustrophobic so we put him in the last stall and he stopped freaking out in the trailer. We never had any problems with him being the smaller horse in the last stall either.
Awesome Miss Take- Storm- 15 yr APHA PHBA Mare
SS Catalena Doll- Claire- 2010 AQHA Filly