KF, I would use a four or maybe a little bigger. The biggest issue will be that your brand should not have any closed area like a "box" or a "6" or"9" especially if the brand feature that is completly closed is pretty small. The hide will slip off on the inside of the closed area and you'll have a mess on your hands.
I have that problem and am in the process of making a new, larger brand so that I won't slip the skin off. My brand is N BOX T where the box is around the T but it is only about 3" across. I have several cows where the brand looks like N and then a big blank spot where the skin slipped. Make sense?