You are probably best taking her out to try on gear from different manufacturers.
My daughter is older and bigger but after many try-ons and online returns, we came up with an adult Mizuno helmet, intermediate Schutt chest protector, and Rawlings intermediate shin guards. All the major manufacturers make youth size gear.
The smallest catcher's set that I have ever laid eyes on was at this past year's NSC clinic in KC. Combat makes the set and they are reasonably priced with removable pads in the shin guards.
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Here's the LINK to the NSC picture and review from January of Combat's smallest catcher's set.
-Chaz