I happen to have a "56cm" Mixte. It's huge, relatively speaking, takes the same fork as my 56cm road bikes. In road setup, it fit me just fine, but my wife can't begin to fit on it. I've got a short 65mm stem, dropped the saddle to the top of the post, and am using upright comfort bars with MTB controls. She sits basically upright. I don't know what I'll do if the reach is still too long once I get it done. It's longer and taller than her 16" MTB, by far. It's also at least 10lbs lighter, so she wants to give it a whirl before I punt and sell it.

They're out there. I bought it thinking "how big can a mixte be?" Pretty big, I find.