You can't really set the size of a player. What you could do however is make it look from their perspective as if they are "1 block tall". This could be either done by setting their actual position under the position of where they should be. That way they'll be in a block but it'll look like they're small. This however prevents the player from actually moving. Another method would be to force them to crawl. With this you'd have to deal with the fact that other players would see them crawling but as you're using mobs you could just set the player model invisible and attach the mob somehow. Plus the movement would also be altered, however, not completely disabled as with the previous method. Unfortunately there really isn't a better way (I can think of) to create this effect without changing the client's code.
