I am about to buy my gym ball off ebay but was wondering about which size. I know the guidelines are those who are 5ft 8 and above should buy a 75cm one instead of a 65cm one. As I am exactly 5ft 7 and a half inches, I wondered if going for the larger ball would be the better option as I also have fairly long legs. I didn't want to get one that is uncomfortable to sit on so just wondered whether anyone around my height had any advice on which size to get. Thanks