9' door MINIMUM. Go with a 10' door if you can. (height)

And you need to bank on boat length plus 7' feet to rough estimate a new boat fitting in there.
So a 353 Formula or a 343 Cobalt is NOT likely to fit in 40'. And of course 40' is never exactly such when you figure in door setbacks, drywall/wall thicknesses; and other infrastructure.
Go with 50' to bad safe it at all possible. Or, make sure it's a storage room that you can rob from directly behind the garage, an NO plumbing or electrical in that wall.

If you have 15' ceilings consider making at least one door to clear a possible future rv, 13' minimum.

hell make it 20' you might get an airplane...

Just had a 16x10 Door installed on my garage. The boat would have cleared just barely with a 9ft door, but went with the 10 anyway.

I had about 43.5 feet inside length, wall to wall. Only thing that saved me, is my boat fits the trailer. I dont have a ton of extra trailer tongue out there away from the boat.

is that what they call pole barn style construction ?? ^^^

yes

I would second a 10' door. It will be tall enough to put pretty much anything in it, without it looking like a 14' RV door, I'll look normal. Good luck.

Appreciate everyone's feedback. I told the builder to go as big as possible. It'll probably end up 10'. Hopefully this thread will help some other folks with planning.

I built my custom/architect designed home in 1979. Everything that went in was custom with special spaces and wiring. I'll guarantee to XX14 that as soon as the home is complete your things will change and the best laid plans will not work. My advice is build it twice as big as your anticipations. You can't make it bigger, but you can always make it smaller. Check weathergod.com/home

10' may not cut it when we all end up with one of these:
(It doesn't fit simply with the arch folded down either.)