Next time take a 3/8" open wrench with you and measure the butt end with that. You also have to only harvest 2nd year growth or older. The first year shoots have leaves (leaf scars now) but the second year small branches grow from last years leaf scars. So any shoots with small branches should be appropriate.
Also with rose shoots, don't peel the bark until they are dry or they will check.
Have you checked the How To section or done a search for rose shafts. Mickie Lotz used rose shafting some I believe and probably did a how to.