The below points are the critcal spots to measure when thinking about a new harness. Print out the below image and write next to each of the letters the size of your horse / pony. Doing this will help both you, and your harness supplier make sure that you are getting a harness that will fit your pony well.
Remember - horses and ponys will flucuate in size throughout the season, so make sure that you get a harness that can go up and down as your pony does.
How to measure a horse when purchasing a harness