Height of Maine Coons

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I've been researching Maine Coons for about a month now, in preperation for a couple of MCs I plan on building a nice big tree for them. I cannot seem to find the average sitting height of these cats. I keep seeing 18 inch height, but I think that is standing height and I think sitting height will be more. I want them to be able to have plenty of room to sit and stretch on the different levels without feeling cramped. Can anyone help me out with this dimension?

Also, what's the approximate standing height of a 14 week MC? I plan on getting a Glacier Point for Cats fountain, the large one, and don't want it to be too tall for kittens. Thank you for any help.