You can print an all-over or jumbo off all the sides of the shirt.
For back prints I've seen 17.5" wide x 22" high for all sizes M-3XL on one particular brands design.
Funny... I ended up here on this same post too for the same advice. Only to end up giving my own observation.
I would say its dependent on the tee your using. Look at the HPS length of the smallest and largest tee size your printing your design on... then use that as your basis. Same for chest width. As long as you can find a printer with a large enough screen the tee size is the limit.