Most oftern resizing of graphics is only done for really small sizes like kids. It's too costly to make screens for many different sizes of the same graphic not to mention too confusing to make sure you have the right ones together. Unless you are doing vinyl transfers. Then it's not as much of a problem.
Thanks.. Found some t-shirt design in templates, I'm probably going to use for the time being! Thanks Again!
Contact the screen printer, they might have a spec sheet or existing templates you can use to go by. Larry's right, you have to come up with a size that will fit their screen parameters. And, if you're planning on adult M - XXL, then one file will suffice for all of those size shirts. You have more flexibility going vertical, but horizontally you're looking at approximately 12 - 15 inches max. ( give or take an inch ). But again, contact the vendor.