I am currently looking for a 7 x 14 so I can haul two full dressed Harley's, I have a 6 x 10 now with interior height of 6'2" and it's just getting to difficult putting both Harley's in there.
When I started getting quotes on them I was told by several dealers that the 7 x 14 was only $100 more than the 7 x 12, I'm going to have one built so it will handle the weight of three bikes (just in case), so it will have tandem axles, most single axles will have a maximum GVW of 3K, floor joist will be 16" on center and interior height will be 6'8", as far as which manufacture to go with, there is so many that I would suggest finding one with a 5year warranty.
Good Luck with your search.