The logo itself effects the sizing. I had a customer that wanted a back print as wide as possible and it was only 9in wide because the design was so long.
For emboridery we try to keep a left chest at 3" but no more than 4". Again, the logo plays a part in the sizing and what the customer wants.
When we print on the back it's never more than 13" wide bacaue my platens are 15" X 15".
Also, if the customer only wants to pay for one screen and they have small-5X shirts, we make it as big as possible for the small and it will look small on the 5X.