This is a basic guideline.  An important factor when deciding on the proper size of a baseball glove is both the age of the player and the number of positions they will play.  For a youth level player that will play multiple positions in a game (INF/OF), consider buying an 11.5 to 12 inch glove.

Baseball Glove Size & Common Use (by Position)
Size	Position
13 inch	Outfield
12.75 inch	Outfield
12.5 inch	Outfield
12.25in	Utility
12 inch	3B/SS
11.75 inch	3B/SS
11.5 inch	3B/SS/2B
11.25 inch	SS/2B
11 inch	2B