Catcher's mitts are sized by circumference, not heel to web: about 30.5" at 8–10 (30–31"), 31.25" at 11–13 and 34.5" for 14 and over (32–34.5"), from Baseball Monkey. Adult fastpitch catchers run 33–35", and a mitt a young catcher can close beats a bigger one.
A mitt measured round the rim
Rawlings publishes no catcher’s row; Baseball Monkey’s baseball, ages 14 and over band is 32–34.5" of circumference, and the sizer picks 34.5". Fastpitch mitts in the same band run 33–35".
A catcher’s mitt has no separate fingers to measure along, so its size is the distance round its padded rim. A mitt too big for a young catcher is slow to close on a low pitch.
The adult game
High school, college and adult baseball is played on the 90-foot diamond with a 60 ft 6 in pitching distance, and gloves are adult patterns throughout.
CDC growth charts put the median 18½-year-old boy at 176.4 cm (5'9") and girl at 163.2 cm (5'4"); the 5th to 95th percentile for boys runs 5'5" to 6'2". A tall player at the age is often a size ahead in the hand too.
Check the hand
Rawlings’ batting-glove chart puts hands at 4–18 in Adult Medium (7.25–7.5") and Adult Large (7.5–7.75") and Adult XL (7.75–8") and Adult 2XL (8–8.25"), measured from the base of the palm to the tip of the middle finger. Measure it 1:1 in Mode 01 (finger plus palm): a longer hand moves the pick toward 34.5", a shorter one toward 32".
For comparison, the first-base mitt sheet starts from 13".