I'll declare upfront that I have an interest in answering here, as I designed my own mounts for Sprat & Winkle couplings, and make them available to anyone else who would find them useful. To answer your two specific questions:
1. the two sizes have different purposes. The loop for mounting the hook is robust, the loop for coupling is as fine as can be usable for reducing the visible intrusiveness. I use 0.45 mm and 0.33 mm straight brass wire, respectively.
2. for making the loops of a consistent size, I've etched holes onto the edge of the fret of mounts. That means they are always consistent in size. For the final fine tuning of the height, I also use a gauge based on a buffer stop.
On both these points, you'll find a more full explanation in the attached file of instructions. I hope they are comprehensive enough to answer your questions, as I view good instructions as a right and not a privilege from kit manufacturers. The two photos show the finished articles, ready for fitting to a vehicle.
Mounting plate for Sprat.pdf