I usually resize my artwork to the following:
Youth Small-Youth Medium: 6 inches
Youth Large-Adult Small: 8 inches
Adult Medium: 10 inches
Adult Large-Adult XL: 12 inches
I use
DTG, so it allows me that versatility on the orders. You could probably get by with 7 inches or thereboots for youth sizes (and adult smalls, which is very similar to a youth XL), then 11-12 inches for adult mediums and up.