You realize of course that a question like that begs two different answers...the literal actual size of the puppet itself and how tall it is in "TVLand" where one allows for the suspension of disbelief (ie if all the Muppets were actually the size of the puppets, even considering feet, most would come up to a human's knees instead of their upper body as is usually portrayed on screen.) The answer to the second question of course is "whatever's required for the scene", though there are some general proportions each character has to each other...and what was great about the Palisades line of action figures is that they tried to honor the proportions the characters were supposed to have to each other in this "TVLand/suspension of belief" realm.
So someone who was really fanatical with both the Muppets and math could probably work out character heights by comparing action figure measurements and using Sweetums as a reference point (since it's usually assumed that full body characters like Big Bird and Sweetums are the same height in reality that they're quoted as being onscreen).