We've put together a handy little guide that will show you how to measure your new (or not-so-new) pony.
Firstly, you need to know where you're measuring to and from. This will depend on your region. If you live in Europe, they tend to measure the back seam from the withers to the tail. In the UK/US, they will measure the chest and the rear. So, taking both of these measurements will help you understand the rug sizes in the store or online. Generally, depending on your horse's size, you will find that it will fit a certain size rug, but as you know, your 17.2' skinny thoroughbred, although it will still be in a big rug, might have a shorter back than your 16.2' heavy breed and therefore be in similar/smaller rugs. Simply knowing the height that corresponds to the size does not guarantee a good fit.
In short, it's best to measure your horse from both of those and then buy the rug that suits your horse the best against both of those measurements.
Once you have your new outfit, it's best not to chance it. Try it on with a clean sheet underneath, that way you can return the rug to the store clean and in resalable condition. Most stores don't ask questions if you keep it clean.