We built ours 10' wide x 12' deep , if you put something in to stop them from moving side to side it works okay, if not its to small and even calves can get you cornered. If I had it to do again it would be 16' wide and at least 12' deep. If you wash big bulls and heifers you have some water splashing in the isle even with 12' deep. It's like every barn, garage, trailer and man cave go bigger than you think you will need. If you are planning your barn and planning on 10x12 pens don't do it, go 12x12, keep them square makes panels, gates, etc work a whole lot better. Those are the mistakes we made and is now to late to change, we like our barn but a few little changes would make a big difference.