With the Firm and Reebok back in the late 80's/early 90's ALWAYS used 10". Found Cathe did 10" and 8" - got a little older, started having bad foot pains and this was about the time Cathe started using 6" on her videos so I switched to 6" also (you KNOW you have to follow the leader) and I haven't had a single (knock on wood) foot or leg pain since.
I remember pushing myself to stay at 10" and then I'd miss 2-3 days working out because of the pain. It would ease and then I'd go right back and do the same thing again and push myself throught 5-6 workouts and then have to layout again. For some insane reason I believed that if I didn't workout at that height I wouldn't get a great workout.
Thank goodness I finally gained some sense and FOR ME the 6" works great. I do a lot more explosive moves now and get my heartrate up as much as I used to. I don't have to layout because of foot and leg pains. With 40 approaching I'd better be wary, but Cathe doesn't seem to be getting any warrier!!
Sami