Mine tend to be between 15" and 20" at birth or shortly thereafter. My tiniest was a preemie born at 283 days. We didn't measure until she was a few days old and already in the hospital, but at about 7 days she was 11lbs and 13". The vet estimated she was born under 10lbs. At 6 months, I could still pick her up. She measured in at roughly 21" at Nationals at 6 months of age. The stick only went as low as 24" so the steward was guessing based on how far the stick was above her body. She's 4 now and is about 27" tall. And nope, not a dwarf, not even close. Managed to earn a Top Ten at Nationals as a weanling in a class of 23 or more and she was by FAR the tiniest.
My tallest came out ALL legs and scared the heck out of me!
I was so shocked at what I saw, I measured him before he even stood (which I now realize doesn't tell you anything because their legs/joints compact as they stand for the first few hours) but my initial measurement had him maturing at 40" tall!
Of course that didn't happen. At 2 years of age, he's about 33" tall and still all leg, but certainly the tallest horse I've ever had born at my place and hope I never break that record!