I just got a notice from the ADS (American Driving Society) that they have put up a new sizing chart for the Very Small Equine (VSE) and Mini equines to help people fit a cart to them. It looks pretty easy to use. The Pleasure Driving Committee has put it together to make it easier to fit the little equines to vehicles that will work for them. I would believe the new chart will be found in the Pleasure Driving section on the ADS website. They say it will be in the New To Driving link, which I found at the Home page, top right column of links. I did not find the measurement link, so maybe it is not posted on the website yet with the holidays. If you are on FB you can find it on various driving groups, like the American Driving Society FB page, there as well.
There is a note that vehicle weight, passengers, fitness of equine, terrain you drive over, can affect the fitting and vary depending of the conformation of the animal pulling.
This chart should be quite helpful to people getting into driving small equines. We have had charts to fit larger ponies and horses to appropriately size carriages for a long time.