There is no hard and fast rule on that. It really all depends on your design. Some design display better the larger they are, many do not.You say "maximum"; is the maximum width also what you consider to be the ideal width in terms of appearance?
Precisely, I concur. The sizing really depends on the design. What you can do as a test prior to actually printing onto a shirt, is to print the design on inkjet paper, cut it out with scissors & arrange it on top of a shirt. It's really up to personal preference how large the design should be..splathead has a good point, sizing can be unique to each design.
For example, I run most artwork at 12"-13" for Adult Large shirts - unless it's circular in shape. Circular artwork (arched text, sports with "balls" lol) come out looking too large at 12"-13".
It's all about context, bro!