I have 6" but it's a Guts Racing seat. The top width varies from 5 to 5.5 inches, front to back. Lower width 7 to 8.5 inches.

 

My experience on sizing of the sweetcheeks on a couple bikes: Buy maybe half an inch smaller than the width of the seat. You want the sleeves (whatever is in the sleeves) to be slightly higher than the seat when you're not sitting on the bike. That way your weight will drop the outriggers down into a level position and provide more support.

When I went with the actual wdth of the seat it was level when I wasn't on it and that allowed the outriggers to drop down under my weight.

Either way, you can make it work buy just sewing an overlap into the center portion if it's a bit too wide. Easy enough to play with it and make it right.