Our Impact 351 has a 12'6" garage and is 38'1". We don't need 12'6" but want to keep this hauler for a long time, so we bought bigger than we need in case we go bigger with the toys later. And yes, account for a bit of space loss if you have the rear patio.
We had the "fits snugly" issue with our last hauler... garage was 9' (no rear patio) and so was our RZR. RZR had to sit against the front wall (it was a solid door to the garage vs glass slider) in order to fit. I felt like it would eventually cause problems with the wall since no matter how tight you tie it down, it still does move a little.