Does length really matter? I mean, for length all you would have to look at are his torso and head.
It does. Like I said, look at Omega Ridley. If all you did to him was make him a little shorter, he wouldn't be too tall - but he'd be very long.
I just think that to make Ridley a regular playable character, you'd have to do too much compression.
I think it comes down to wether or not you think an awesome yet under-represented series deserves to have its primary antagonist in. If not Ridley, then Dark Samus. I doubt Dark Samus because Dark Samus was only the antagonist for two, three games. Rundas and Sylux are possibilities too, but they were one game a piece.
Plus, I've heard Sakurai hates Metroid Prime.
Metroid certainly does deserve to get more characters, but the problem is that there's not really many you could use. We've discussed Ridley. Dark Samus, if you were being accurate to the game, wouldn't be very similar to Samus, but if you put it in you have the problem of having a character who not only looks almost just like Samus (even moreso than Ness and Lucas), but has the same name as Samus but with "Dark" added in front of it. No matter how original of a moveset it had, Dark Samus would look too much like a redundant character.
And there's no one else who could fit in, since the only other real candidates (the other hunters) are far, far too minor (and most or all of them will probably never appear again). If Sylux manages to reappear and become important in future games, then he'd probably be the best Metroid candidate, but otherwise I see Samus going it alone.
Sakurai definitely doesn't hate Metroid Prime (or Metroid in general, look how much non-character representation it got compared to a lot of other series). Japanese gamers don't really like the Prime series much, though.
Remember that Sakurai said he wanted to include more Metroid characters in Brawl, so he included Zero Suit Samus. When he specifically says he wants more Metroid characters in, yet his only pick is another version of Samus, it shows that he also feels Metroid doesn't really have any viable characters.
EDIT: Also, Olimar doesn't look right scaled up. I question Bowser scaled down. I also question Kirby scaled up. They all look weird compared to several other characters.
Except that they don't require compressing or stretching of certain body parts that makes them look awkward. There's a difference between just changing someone's size and what you'd have to do with Ridley.
and even sonic is diffucult to imagine in that kind of game it's a good thing it was sonic not someone else then i would feel mad. 
