We recently brought in four OTTBs and two sucklings from the Sam Houston Racetrack. All six horses were crammed into three nearly standing sized stalls (10 by 10 I believe?), incapable of moving more than a few inches. Two have severe thrush, all of their muscles are atrophied, and one has lung problems due to the ammonia produced by all of them. It's sort of a 'worst case scenario' but it does show that bad ventilation and no chance to stretch out and do what horses do can ruin a horse.
So, good exercise, access to pasture, and good ventilations and I'm good with a standing stall. Otherwise, I'd say just put in the extra money for a box stall.