Looks like you should be safe with 50 feet until you measure it! lol

I believe mine is 45' on an aluminum Myco.

My 38 Donzi is a true 38' long. My Loadmaster aluminum trailer has roughly 5' of tongue length and the Its Xr's are 4' off the back for a total of 47'.

Just parked mine last night. 46ft w steel Manning and boxes- NOTE build 60' for when you have to take off drives! Not that That will happen

Boat plus 7 feet is a good estimate. It all depends on the trailer though.

Myco and other welded trailers tend to be shorter with the boat sitting closer to the hitch. Bolt-together trailers tend to put the boat further from the hitch.
Surge brakes add about a foot over EoH due to the coupler.
Stand-off boxes or ITS drives, add another foot or so.
Surface drives, add 3 feet.

If you are planning for a building, don't make it any shorter than 60 feet. I recommend 75. That way you can park your boat and truck on the same side of the building, and NOT have to unhitch it when you get home at 1AM. It really saves a lot of parking space parking your largest vehicles on the same side. Go as big as you can; and then double it. (Lots of posts on buildings here on OSO. I built my barn when I had 1 boat and 1 truck. Now, I have a lot more crap that I like having inside.

There are option to reduce the front tongue length.... If already have fixed space would be much cheaper to modify trailer than to increase garage.....

My old one on a manning gooseneck was 48'.