There are so many different brands, styles, etc. Do you think she would need something really long or would a smaller "teen" version work better?
Lots of brands come in a "slender or teen" version. rhythmandbluespads.com has a teen/slender pad that's narrower than their regular ones and only 8" long, Newmoonpads.com offers some 8" versions with 3" width, Punky's Pads ebay store has some 7" length ones that are only 2.5" wide, Snap-EZ4life.com offers a 7" mini and I hear they are very narrow. Lunapads.com has a 6" long X 5.5" wide Teeny Pantyliner version. HagRag.com has 3" wide pads that go from 6.5" long to 11.5" long. TLC-Pads.com has pads from 6" up to 12". The list could go on and on.
Regular pads (non-Teen versions) can go from usually 8" all the way up to 16" for a nighttime pad. The size DD needs depends on her size and how heavy her flow would be. I'm a pants size women's 6 and I find 9.5-10.5" to be good for daytime use so she probably wouldn't need any of those huge 16" pads at her age, they'd be way too big!!!
I'd get her a few different versions and let her tell you after she's used them what she likes or doesn't like in a pad, that way you'll know what to look for when you buy more.