She's very pretty. I definitely see a GSD. And at nearly 50 pounds and 24 inches, she's an average sized female. Her coloring is pale but that doesn't in any way indicate she is not a PB dog.
Of course, without knowing her history, where she came from, etc, there's no way to know 100% for sure that she is or is not purebred. To me, however, I'd say she is a purebred dog, likely from a BYB, and very lucky to have found you. She's beautiful and looks very happy. Enjoy her!
Something to consider, as well...In our area, shelters will often list purebred dogs as mixes if they don't have proper documentation to prove the dog is purebred.