I’m showing 1st level (recognized) in a few weeks and I only have a snaffle bit with the bradoon ring. Is this legal or can you only use the full size ring?
You’re fine. I used a bradoon snaffle on my fine-headed gelding. As long as the mouthpiece is legal, no problem!
Does it say that anywhere in the rule book?
You are fine to use a bradoon as a regular snaffle. The mouthpiece cannot be smaller than 10mm. If I remember correctly the ring cannot be smaller than 40mm, Most bradoons are 55mm, the Neue Schule Pony bits have a 45mm ring and they are fine to use also.
You can read all about bits in DR121, and the illustrated supplemental equipment guide: https://www.usef.org/forms-pubs/sAH3nOVD85c/dressage-attire-equipment-booklet
As far as I’m aware, there are no rules about the diameter of the ring on your snaffle. You can use whatever suits your horse best.
the only restriction on the snaffle is that it is 10mm diameter at the rings. A bradoon is only a bradoon in association with a curb bit, alone it is a snaffle. As long as your bit complies with all the aspects of a proper snaffle ( smooth, joints , bushings and rollers) , you can use it. Figure 1 is your reference
No restriction on size of ring, just on bit.