I don’t have an explanation for the sizing problem. It sounds like the Epics fit–they should be a bit hard to get on. I don’t use the straps, either.
Look at the instructions for the cable positions, because you can hook it on the prongs or have it just go between, depending on how tight you need it. It’s best to have the cable tight enough that you need to step on it to close it. Otherwise it might pop open during a ride. I know that is a pain, but if you can close the clip with your hands, the cable is too loose.
I keep a nice hoof pick, a solid metal one, to pop the clips open. I attach one on my saddle when I ride in case I need to remove a boot. I also use the hoof pick to move the wire around, since they are hard on fingers. I put the boot on, then hook the cable on the prongs (depending on the horse, I might only have to hook one side of the cable over), position it in the clip, and step on it to close.
It’s really not as hard as it sounds, once you get the hang of it. I can install and remove boots pretty quickly these days. Had to leave myself plenty of extra time in the beginning (and make sure no children were around to hear the cussing).