Since you're not having a sanctioned show, you will not need to follow registry rules exactly, but you could definitely look to them for guidelines.
In AMHA, the jumps have to be 5 feet wide. Being 42 inches, yours are probably a little bit small, but the horses should still be able to jump them. If you have a choice, though, you should probably make them wider.
There are two different jumping classes, Hunter and Jumper. In hunter, the horse's way of going is judged, and the horse must keep an even pace throughout the course. The AMHA rulebook says that jumps cannot be higher than 24 inches, but you will never really see hunter courses with jumps less than 24 inches--all the jumps will usually be that height. For your class, a jump 12 inches high is likely too short.
Jumper is judged solely on whether or not the horse makes it over the jump. In AMHA, the focus is on height--jumps start at 24 inches and will be raised to well over 40 inches for the top contenders. In AMHR, the focus is on speed. The jumps aren't raised very high, but the jump-offs are timed, with the fastest horse winning.
You should definitely decide whether you are going to have a hunter class or a jumper class, as they are quite different and the exhibitors will want to know what they're entering. Regardless of the class you decide to have, you will need to have at least 5 different jumps to use, and the pattern should consist of 6-8 jumps (so some jumps will be jumped twice.) I'm sure you will find reading the associations' rulebooks helpful. The AMHA rulebook can be found at www.amha.org, and I am pretty sure that the AMHR rulebook is also online at www.shetlandminiature.com.
Good luck with your show!
