Before you buy, check around town to see what size bonnets are being stocked. EVERY store here stocked a wide variety of 7-8" bonnets for at least a decade, then one day - as if someone flipped a switch - they all dropped that size and changed to 5-6" and 9-10" bonnets.Coincidentally, they also started selling buffers to match.
Of course, if you're good at planning ahead, you can buy bonnets of any size online cheap enough that you pay less even after shipping.
That being said... I actually prefer the smaller buffers. A larger one may cover a little more area, but the smaller sizes make it so much easier to get into tight places and concave curved areas.