This very helpful video is from Tshirt Help Desk
Published on May 15, 2013 and is 5:15. His website is https://tshirthelpdesk.com/
The author recommends 8.5×11 for women and 11×17 for men as the size altho you don’t have to fill up your whole area in either case.
Anything larger than 10 to 11 inches won’t work and author explains why – if image is larger than that both lateral sides will be on the side of t shirt when worn.
The best way to tell if our artwork actually works is to cut the artwork out of our 11×17 print and then place it on the t-shirt.
9:25 If you want to send your artwork to a printer, you have to tell them where to place it on the t shirt. Tell the printer you want the artwork centered and the top edge of the graphic the distance from the neck edge or collar edge.
In the author’s example the top edge is 2 1/2 inches from the center front collar edge.
11:30 The author shows us how to mark the graphic we send to our printer.