Placement and scale do more for how a printed shirt looks than the artwork itself does. A good design badly sized reads as homemade; a simple one sized well reads as a brand.
Chest prints are narrower than people guess
A full-width chest print on an adult shirt is usually somewhere around ten to twelve inches across. Much wider and it runs into the side seams, where the press cannot lie flat and the print distorts. Much narrower and it starts to look like a left-chest logo that has drifted.
Left-chest logos are small
A left-chest print is typically three to four inches wide. It looks alarmingly small on screen and correct on a body, which is the opposite of what most people expect when they first see the proof.
Youth and toddler sizes are not just scaled down
A design sized for an adult shirt will run off the edges of a toddler tee. Children's garments generally need the artwork re-sized rather than simply printed smaller, and sometimes need detail simplified so it survives at the reduced size.
Height on the chest matters as much as width
Prints usually sit two to three inches below the collar seam. Higher creeps toward the neck and looks cramped; lower disappears under a belt line when the wearer sits down.
Ask to see it on the garment
A design that looks balanced on a screen can sit oddly on a real shirt, particularly across different sizes. If you are ordering a range from toddler to 5XL, it is worth asking how the print will be handled at the extremes rather than assuming one file covers everything.