Diameter i've always known as 7/8" on all of my bikes, which would be 22mm roughly. From memory it's always been touted that bikes like Harley's had the 1"diameter, or 25mm.
I've only ever fitted heated grips on mine so just ordered them per bike and not taken any notice on length, but why can't you just measure the old ones, or as said just order ones that fit the FZS600?