I used some 4x4 designs on the upper left chest area of some of my shirts. To find the right placement, I tried the shirt on and marked where I wanted it to go. You don't want to put a design in the wrong spot on the chest, if you know what I mean!
There's software called Place ahnd Stitch embroidery that has 400 stitch outline templates. It makes placement super easy. Nancy Zieman from Sewing with Nancy developed it.