Well as shown in the episode "A Friend in Deed" it was revealed that the ponies are relatively the same size as donkeys, and the average "American Donkey"(that's a breed of donkey you understand) is measured about 4 feet tall at the withers(shoulder blades), of course you take into account that the male donkeys are taller than the females, now I'm not sure exactly what breed the ponies & donkeys were based on or anything, but my belief is that these "Little Ponies" are averagely around the same sizes that donkeys come in, so I don't believe that they are itsy bitsy or even gigantic but closer to the size of actual ponies depending on their breeds.