The world around them seems to be different than ours. They have been the size of grass, a book, a puzzle, etc. But the airplane, boat, houses stay relative to their size. I always pictured them as being 2 feet tall though
I always imagined them to be versatile to the situation. Quiet dice sized key chain danglies to monstrosity protective humongo rawer! sized. It's part of the whole they are whatever gender fits you, they're whatever size they need to be at the time.
I'm going to say between 2-3 feet, because in Reeeeeach, most wooden posts like that are like 5-6" so then the chain, it would be about the proper size of a medium-large dog. And in Wake Up, Pon's head barely comes over the bottom of the window-- which makes me think 3'. So yes, somewhere between 2 and 3 feet tall.