I have the small cruiser on my 1150, at 12" wide it is wide enough without overhanging and doesn't get in the way of your legs.
But more importantly it is 18" long front to back which allows you to move about a bit. I tried the medium cruiser and it felt too short front to back, like I was about to slide of it all the time.
If you think you need something larger though then the Large Cruiser might be best as it is both wide AND long.