At 6'7" I wear a "3xl Tall." When I was Fire Chief we had a lot of folks on the department that needed 3XL-5XL. The local screen printing shop owners were close friends so we did a lot of experimenting. Found out we were wasting time doing multiple sized images for the front of shirts. Most people can't tell the difference and if you make it real big the person looks like a walking billboard. . . and not in a good way.
The back of shirts was on a case-by-case basis. If it was a short but wide graphic then it sometimes needed to be made bigger on the backs of the larger shirts. This was especially true if it was arced. Arced text on the back kind of needs to almost touch the shoulder blades to look good and when you start looking at someone my size that's quite a bit wider then a normal XL.
You also need to take the weight of all that vinyl or ink into consideration. The wider it gets, the more it wants to make the shirt sag. When you scale up, the thickness of every stroke gets larger too, so if you put a 14" wide graphic on the front of a shirt there is going to be a lot of sweating going on.