If you're over 5ft 6 it is recommended that you use the 75 cm- it is to do with the height of your pelvis compared to your knees, so if you're on a ball that is too small then your pelvis will be lower than your knees, thus not the best position for baby or fitness exercises.( I teach sports therapy!)
I find that if you're on a ball that is slightly to small for you- sit more forward on it, rather than centre on it- and you should be able to get your pelvis higher.
I have the biggest one too, as I am 6ft.