I did an 8 x 12 many moons ago. Straight forward 2x4 construction and I did a high peaked roof to be able to loft some things that got used very rarely. It got cramped very fast and basically felt like an overstuffed closet by the time I got my gear in there. I wasn't ever really able to lay things out and work on them. I suppose it depends on how much of a gear junkie you are. I'm guessing pretty bad since you have multiple boats and are looking for a dedicated shed
I'd say to go as big as you reasonably can and not worry too much about keeping it in 4' increments. If you have room to squeeze it to 11' wide then you already have some 12" wide shelf material on hand.
I'm just now finishing up my new shop and am building it with a high and deep bench so that I can fit full size items like dryboxes and coolers under it while still closing a door to keep it neat. You'll never say to yourself that you wished you had a smaller bench top to work on. I'm laying everything out for "rapid deployment", meaning that I'll be able to back the trailer up and quickly load dryboxes, ammo cans, propane, stove, groover, cooler, etc.