These size recommendations are an average and may not pertain to every player. They will give you a guide whether you are a Baseball, Fastpitch softball, or Slowpitch Softball player.
First off, how is a glove measured?
Fielders gloves and first base mitts are measured by starting at the top of the index finger of the glove down the finger along the inside of the pocket and then out to the heal of the glove. Lay the tape measure across the palm of the glove, so that it folds across and into the indenture, down to the heel of the glove. A catcher's mitt is measured by the circumference.
Adult Baseball Gloves
Outfielder gloves are going to offer a larger pattern size in comparison to infielder gloves. This is due to outfielders needing a larger pocket to assist with pop flies while infielders need a smaller, more shallow pocket to help with quick transitions. But, ultimately, the size of your glove comes down to what is most comfortable for you. The best way to find that out is trial by error.
Second Base & Shortstop Gloves: 11.25 - 11.50 inch pattern
Third Base Gloves: 11.50 - 12.00 inch pattern
Pitcher Gloves: 11.75 - 12.00 inch pattern
Outfield Gloves: 12.25 - 12.75 inch pattern
Fastpitch Softball Gloves
Due to the larger softball being used, Fastpitch softball gloves for both infield and outfield feature a larger pattern in comparison to baseball gloves. But, similar to youth gloves, a lot of models will feature a smaller wrist opening and tighter finger stalls to accommodate a female athlete's hand. Typically, these softball gloves range from 11.75 inches all the way up to 13 inches.
Infield Softball Gloves: 11.75 - 12.50 inch pattern
Pitcher Softball Gloves: 12.00 - 12.50 inch pattern
Outfield Softball Gloves: 12.50 - 13.00 inch pattern
Youth gloves are recommended for younger players who have not stopped growing and are likely to outgrow their glove sooner rather than later. The age range of youth players is 12 years old and under. Because of the wide range in age, the sizes are going to have a larger range too. The difference in youth gloves is in the wrist opening and finger stalls. Manufacturers, such as Wilson and Rawlings, craft their youth gloves with a smaller wrist opening and tighter finger stalls for a more secure fit on a youth player's hand.
Infield Gloves: 9.00 - 11.50 inch pattern
Pitcher / Outfield Gloves: 10.50 - 12.50 inch pattern
Slow Pitch Softball Gloves
Slow Pitch softball gloves are in a realm of their own. Slow Pitch softball gloves tend to run much larger in comparison to both baseball gloves and Fastpitch softball gloves and range from 13 inches all the way to 15 inches.
Infield Slow Pitch Gloves: 13.00 - 14.00 inch pattern
Outfield Slow Pitch Gloves: 13.00 - 15.00 inch pattern
First Base Mitts
To be able to play first base, you need to be able to catch extremely well. Having the right sized mitt helps. First base glove sizes will range from 12 inches to 13 inches.
Baseball First Base Mitts: 12.00 - 12.50 inch pattern
Fastpitch Softball First Base Mitts: 12.50 - 13.00 inch pattern
These gloves range in size from 31 inches to 34 inches. This isn't because they are three times the size of a normal glove, but rather because these mitts are measured by their circumference rather than top to bottom.
Youth Catcher's Mitts: 31.00 - 33.50 inch pattern
Adult Catcher's Mitts: 32.00 - 34.00 inch pattern
Fastpitch Softball Catcher's Mitts: 32.50 - 34.00 inch pattern