well if shes usually a 5 1/4, you need an inch bigger ideallly so i'd say a 5 1/2 will be too small and pinch x I'd look for a 6 1/4 but a 6 would be ok if they dont make 6 1/4 x
I've not long swapped into a waterford snaffle from a single joint. Think it's feared for no reason. I get a nicer outline (I think she objected to the joint into the roof of the mouth) AND I have brakes when I need them.
Just remember - soft hands.
Also I forgot about the sizing rules and bought a 5" for my 5" mouthed horse.... It fits perfectly....
Personally, I would never use one, though I know many people love them. I believe they damage the bars of the mouth if used by an unbalanced rider, or even an experienced, balanced rider if somethig happens to unseat them. The nobbles fit around the thin bony bars of the mouth. This cannot be a pleasant thing for a horse. It is no wonder they do not lean on them.