There is no one definitive answer, as growth rates vary significantly between horses. However, by looking at a horse's breed and growth rate up to 3 years of age, you can get a pretty good idea of how large they will eventually be.
Foals are typically born around 33-35 inches in height and between 75 and 100 pounds. Foals will continue to grow rapidly in their first year, with growth rates slowing down in their second and third year. Foals born to small mares may be smaller than average, while foals born to larger mares can be significantly larger.
The breed also plays a large role in the size of a horse. Horses with Arabian bloodlines tend to be on the smaller side, while big horses like draft horses and warmbloods are commonly larger than average. However, growth rates can vary within a breed, and so cannot be relied upon entirely.