I know of no 'rule' and, if there was one, I'd probably ignore it anyway. It's all about proportion. With 9" log cabin blocks using ¾" logs, I definitely wouldn't go any bigger than a 1½" stop border. That's twice the log width and one sixth of the block width. It would transition very smoothly to your wider border (which is part of the purpose of a stop border after all). Your 1¼" plan sounds fine though. For the wide border, auditioning is the best idea if you don't have a specific finished size that you want to wind up with.
Last edited by ghostrider; 04-04-2015 at 05:31 AM.