If by "tip to tip", you mean circumference, 24" will work.
As to the bury, I'll go with your gut...I wonder what Frawg used?
Dave, the finished bury was 5" on each side of the lock. Note that I'm not putting all that much strain on the bury. It was just my way of making a loop to use in place of the ones I'd done previously using a Zeppelin bend.