They are all set up for a 14 x 14 cutout. Just go for a an unobtrusive low profile...and go for a 15000 instead of a 13000....you won't regret it.
Ahhh...as I reread your sizing I guess your referring to the length of the units case. I had that problem on a three vent Flying cloud. After..repeat.AFTER....I had everything set for the installation of the Carrier and the unit was on the roof...I found out that the case hit the center vent cover. I had to regroup and cut (modify) the AC hole by 4 in. forward to clear the center cover. That involved cutting and bracing the roof hole. You will also have to consider the drain line.
"If it can't be reduced, reused, repaired, rebuilt, refurbished, refinished, resold, recycled or composted
then it should be restricted, redesigned or removed from production."