Hiya,
Well, I’m certainly one of the ones pushing the limit with this topic. I’m 5’9 and my Welsh cob Cardi is just 14.3, but it hasn’t stopped him from being very successful up through Third level - competing, and winning, in the Open division against other trainers on big “fancy” WB’s.
As stated above, I think there are two important parts of the equation. One, the pony must be a size that your body can ride effectively. Just as a trainer wouldn’t (or shouldn’t) recommend an 18h for your petite self, there is a limit to how SMALL you can go and still feel like you can, quite literally, ride them. I’m quite tall to be riding him, but his deep barrel lets me put my leg on…
Second, they have to move and train the part. Since you say that showing isn’t everything, there’s some flexibility here. A “good” mover might suit you just fine, especially if it’s trainable and reliable. If you wanted to be “very” competitive in the open shows, your mount - regardless of size - must have the gaits, balance, elasticity, etc expected in a dressage horse. It must have the gaits and balance to carry you well.
And I’ll say it again - there is a significant trend toward more appropriate sized mounts and the judges are starting to come around to it. There are a lot of petite individuals with plenty of talent out there winning!! Join the fun…