Doesn't the size of the AirHawk depend on the size of your...ahhhhhh...fundament*?
Actually, no. What matters is the size of the motorcycle seat - you want the device to cover the seat, but to NOT extend beyond the edges of the seat. If it extends beyond the sides of the seat, it will irritate your thighs.
When you get the Airhawk, read the instructions carefully. The trick is to fill it with the
least amount of air needed to prevent any portion of your butt (typically the ischial tuberosity, which is the portion of your pelvis behind your butt) from pressing all the way through the Airhawk. You don't want any more than about a half an inch of airspace between the lowest part of your butt (when on the Airhawk) and the seat. If you over-inflate it, it will not be comfortable. If you inflate it correctly, your body weight will be evenly spread across all portions of your butt that come in contact with the Airhawk, with no 'pressure points' anywhere.
Michael