White box fans work well if you can get them on the stall or hanging well. You have to make sure it's one designed for barn use with an enclosed motor, not an open motor like the cheaper human ones you'd get from Wal-Mart. The ones with the enclosed motor prevent dust from getting in and causing a fire/ the motor to die, you'd find these at farm supply stores. They will cost more than the Wal-Mart variety, but are far safer.
The big black high velocity fans I find are best for barn aisles or very open stall layouts. They will facilitate quicker cooling than the box fans as they move more air in a faster manner. However some horses do not like them and must be made accustom to them. Again, look for ones with enclosed motors.
As for size it really depends on the size of the area, the heat/humidity level, and to an extent the size of the horse. I like a medium to larger sized fan, but I'm in a hot humid area and like as much of a breeze as I can get on summer days. I suggest a medium size to start, about 20 inches or so.
"They see me rollin, They hatin, Patrolling they tryin to catch me ridin dirty"