Big... Bigger...
Bare Minimum would be, IMHO, the 26" or 28" tubular, or the 15" dia. round ones. Larger is better - much better.
The round ones are expecially nice for rafting up with friends' boats.
Braid some eyes into some 3/8" three strand nylon line, about 6 or 7' long, and use 'em for fender whips.
While boating newbies may roll their eyes as you extract large fenders from the lazarette... they will change their opinion the first time they tie up to a dock or other boat, on a bouncy day...
Speaking of tying up those protective sausages, do not tie them to lifelines except on a quiet days as a last resort. You probably know this, but they need to go to the toe rail, genny car slider, stanchion base, shroud... i.e. something that can take a *lot* of tension when the hull pulls hard on the fender and tries to rock back'n'forth. You will have around 10K# of determined Ericson leaning on those "gel coat savers!"
Loren