Understand that a young player needs a glove that he can control. Forget about giving a youngster two or three extra inches from the top of his fingers to the top of his glove because he will not be able to close the glove with any kind of ease and won't be able to catch the ball. The first thing you should look for when purchasing a glove for a young player is one that is small enough to operate correctly. Yes, it must be big enough to catch and hold a ball and that can be determined at the sporting goods store. A younger player should have a glove that is about an inch longer than the spot the top of his fingers reach in the glove.
Know that an infielder needs a glove that is a bit smaller than an outfielder's glove. A player at the high school level and above needs a glove that is at least an inch and a half above the spot the top of his fingers reach. The infielder has to be able to quickly reach into the glove and retrieve the ball. It may be harder to do that if the glove is too large.
Keep in mind that an outfielder needs a glove that may extend 3 inches from the top of the fingers. While a long reach can help an outfielder catch or cut off balls hit into the gap, the outfielder can't have a glove that is so big that you can't control the ball. Typically, a veteran softball outfielder feels comfortable with a glove that extends 3 inches but will struggle with a bigger glove. You can't catch the ball if you can't control the glove.
Remember that a first baseman's mitt in softball is very similar to its counterpart in baseball. The first baseman's mitt in softball should be no more than an inch or so bigger than the first baseman's mitt in baseball. Reacting quickly and controlling the ball in the glove is more important than the ability to reach wide for an errant throw.
Consider that the catcher's mitt in softball should have a distinct hinge that allows the catcher to close it easily. The catcher's mitt in softball is about the same size as the catcher's mitt in baseball but its baseball equivalent is stiffer. A hinged catcher's mitt gives the catcher greater flexibility when receiving the pitched ball.