I layed my shirts out and measured what looked like a good size to me. I then printed out on regular paper my first design, stood in front of a mirror and held it up in front of me to determine positioning. I went visually by the underarm part of the sleeve and took a guestimate on inch(es) above this point as to where to place it on the shirt.
I ended up with the majority of the designs being
Adult - 10.5 wide x 8.25 high (when I printed the test using the entire sheet somehow some of the border got cut off so I went a little smaller)
Childs - I shrunk by 1.5 inches each direction. 9wide by 6.75 high