Not as padded as normal punches, so not recommended for beginners and lower intermediate levels.  Recommended for Brown and Above.
 
Size
Knuckle Width
Diagram
Child Medium
2 1/2" - 2 3/4"
 
Child Large
2-7/8" - 3-1/4"
Small
3-1/4" - 3-3/8"
Medium
3-3/8" - 3-5/8"
Large
3-5/8" - 3-3/4"
X-Large
3-3/4" - 3-7/8"