That's interesting... and kinda inconvenient. Mii Maker is absurdly slow so we really just can't use it in the middle of a tournament. If the Miis have three preset sizes like Mario Kart, we can just have pre-made Miis on each set-up to accomodate this character. By this character I really mean these three characters; suggesting we ban Mii Fighter if it's at all possible to avoid is really not reasonable since it's three entire characters which is a really large and profound chunk of gameplay (if it were Mario, Link, and Pikachu, no one would question how clear it is that banning these characters is a very last resort action). If it's really some continuous spectrum and the game doesn't offer a default, we might have to just agree on a forced standard (IMO the best solution there would be default Mii size parameters out of the Mii Maker), and we'd have to rely on good faith in people preparing set-ups not to doctor Miis to alter gameplay though we kinda already do that with Brawl and Project M given that it's really easy with current tech to load up doctored files for either game to give yourself a sneaky edge (though I've never heard of anyone actually doing it in a tournament).
I'm kinda thinking with all of these customization features and the unrevealed 3DS connectivity that maybe this is the connectivity feature: importing all of these settings saved on a 3DS to the Wii U on the fly. That would make setting a million fine parameters on Miis or picking out super specific customization stuff really fast and easy in a tournament setting though perhaps it's not reasonable to suggest people drop the $150ish to buy a 3DS and copy of smash 3DS to enter Wii U tournaments and not be at a disadvantage. This is kinda a second picture in a row showing something that is kinda not so great, but we'll see how it plays out when we have everything in our hands I guess.
