10' doors are sufficient for pretty much anything. Like stated above, better to go big now than be stuck with it later. If you have the room to put a 12' door in, do it for sure. Also go wide enough you can get something out around the boat if you need to. My old shop had a 9' wide 8' High door and my formula fits so tightly I cant even put it in with the cockpit cover poles installed, and I have to prop the garage door up past the header with a stick so the windshield clears.. Once it's in, I cant even squeeze a lawnmower out around it.
I'm installing doors in my new shop this weekend, 2 14'W 10.5' H and 2 10 x10. I wanted to go higher, but the I'm restoring an old barn, and that's the best I can do.
Dave - I'm glad your Donzi fits in a 9'er... That's one of the boats we have been seriously looking at and I was mildly concerned it would be tight with a 10'. Good to know.