My dd uses a SSK-MP44 34" Fastpitch catchers mitt. SSK gloves are very hard to find these days, and she absolutely loves it.
The typical fastpitch catchers mitt is 33 or 34 inches in circumference. The fastpitch catchers mitts are designed for a smaller hand than the baseball catchers mitts. The baseball catchers mitts are not designed to trap a 12" softball which equates to a very shallow pocket and occasional "snags" from the padded outer edges of the baseball catchers mitt during transfer. It is easier to scoop a dirt ball with a fastpitch mitt. As with most gloves and mitts, you get what you pay for. A good catchers mitt in a sporting goods store will be anywhere between $150 and $300. Top of the line Easton, Mizuno, Nokona, Wilson, etc. are all very good and it really is a matter of which mitt feels the best on your dd's hand.
Len