I use a 7" bladder tube made from a T-8 fluorescent tube guard for my Yankee Nippers. You can get the tube guards at hardware stores (4 foot lengths). It's about 1 1/4 inch diameter, and I use a 7" long piece of tube. This works for OS 32sx and Magnum 32xl for speed limit combat with a 5 minute match duration. If your plans say to use a little bigger, I would trust
that size instead.
-Chris