Probably a 5' Weymouth with a 5 1/4 bradoon. Or a 5 1/4 Weymouth and a 5 1/2 bradoon..... I usually go down between 1/4' or 1/2' for the weymouth, depending on the style and mouth conformation.
Ah, you see, I go down 1/4 for the weymouth from my usual snaffle size as you want the weymouth to sit snugly at the corners of the mouth, not slightly over as you do with a snaffle.