I ordered a blue 75cm ball & base intending to use it as a chair at the office. The box/packaging appeared to indicate that the ball I received was the 75cm size. I did follow the instructions of inflating to 80%, waiting a day, then inflating the rest of the way. I inflated it to be quite firm, however the ball is 65cm rather than 75cm (per the tape measure that was conveniently included).
In my case, this turned out to be the perfect size, and 75 would have definitely been way too big. I love the base that allows me to roll and bounce a little bit while seated without actually having to be worried about rolling over. Overall I am happy with my purchase of the ball and base as an ADHD-friendly seating solution, and it is the right size for my needs.
However, if you are a taller person and genuinely need the 75cm size, be aware that they may send you a ball that is smaller than indicated.