Ok, I remeasured the garage.
The total length of the garage is almost exactly 20'. Width isn't a consideration, but if I take up too much such that my wife can't park her car inside, it is a non-starter. Unfortunately, earlier this year I built a really nice work bench on my side, shortening the usable length by 2'. So any build can be no longer that 18'.
This doesn't preclude building a Tango. I will just have to modify the build to make the hull length shorter than 18'. I'll just shorten the build by 1' (17' - 6"). This is easily within the 10% allowable. It won't affect the boat much... and the difference will mostly come out of the cockpit. Cabin space should be unaffected.