I made a gate in my fence the other day that leads to a cement pad that I had poured in my back yard for my boat storage. I made the opening of the gate 10 feet wide.
The trailer fits, but I only have about 4 or 5 inches on each side of the trailer. That doesn't leave alot of room to play with so I have to be pretty much dead on everytime.
In hindsight, I wish I would have made it 11 feet. If you're going to build a garage, I'd make the door at least 12 feet wide.