Constructed with a large, rounded, oversize pocket, a first baseman's mitt is designed to aid baseball and softball fielders in scooping balls out of the dirt. A common mistake when trying to determine a first baseman's mitt size is selecting a glove based on the size of the glove's pocket. The correct way to choose a first baseman's mitt is to match the glove to the size of the player's hand, to give the player maximum control of the ball.
Hold the end of the tape measure at the tip of the longest finger of the player's hand. For most people the longest finger will be the middle finger.
Run the tape measure down along the longest finger and through the palm to where the hand joins the wrist. Lay the tape measure flat so the tape is touching the hand from the top of the finger to the base of the palm.
Select a mitt to match the measurement of the hand. Baseball and softball gloves are sized in inches, from 9 inches for youth to 13 inches for adults.
Try the glove on the hand to check the fit.