I have an arabian/quarter cross so i'm guessing she might have the same type of face/head as yours (small but not as small as most arabians).
I buy my horse arabian sized halters and flymasks and they fit good.
When it comes to bridles and headstalls, the "cob" size fits her rather then the standerd "horse" size. Having to do with the bit, she fits well into an 4-1/2 inch rather then the standerd 5 inch. Maybe this is what you mean by different bits?
Hope this helps