Yep, Xue's head is larger than the Dai and probably the Dan (because they are very similar, with different ears). A stretchy 7/8 (like Monique Gold, Jpop, etc.) fits the Dai perfectly, but they tend to slip off of my Xue. The 8/9s are a bit big but at least they don't pop off like a little rocket.
My Xue Angel in a size 8/9 Jpop Wigs "Reiya".
Katja (Dai) in Monique Gold 7/8 wig, can't remember what it is.