I believe your little sleigh is modern, and perhaps the maker didn't quite plan for perfect drafT anyway. If memory serves you should be able to draw an imaginary line along the traces, through the singletree, and have it reach the ground at the back of the runners. So aside from the height of the horse, you have options as to singletree height according to the design of the shafts. There are perfectly straight shafts, which would be the lowest singletree height. Sleigh shafts typically have a low heel (bend) at the back, for a slightly elevated singletree. There is an "S Bend heel that raises the draft higher yet, and finally a buggy style single bend heel shaft that would raise the draft (traces) to nearly horizontal. Also, how close you hitch to the sleigh will affect the draft. Getting the draft right will impact how the sleigh runs, and also make easier work for the horse.
Your other concern is width at the back of the shafts.