It makes you wonder if Princess Celestia is actually a horse or not considering she's about 3 average adult ponies high. I know Alicorns are supposed to be tall, but it does make you wonder. It makes me wonder.
Well... let's just say that she's the size of an average horse here around 6-7 feet tall), and assume everyone else is around 4 feet tall as adults. They are ponies after all... not legit horses as her.
That's a really good question, but like her origin, we'll probably never get an answer to it ^^. Maybe alicorns are neither pony nor horse, but something completely different... who knows, more space for creative theories xD