Of course he could be put in the game. I'm in no way saying it's impossible, but he won't be playable. Nintendo has done a pretty good job of keeping him consistantly huge for over 20 years, I don't think they want to break that streak.
Actually, they have broken that streak tons of times by now, Ridley has been nearly as inconsistent in size as Bowser (not including spin-offs.)
In Metroid 1 he was tiny.
In Super Metroid he was suddenly huge.
In Melee's intro he was barely taller than Samus, but his wings were huge.
In Metroid Fusion he was even bigger than in Super Metroid, but his wings were smaller than ever.
In Metroid Prime his body wasn't too big, but his wings were enormous.
In Zero Mission he was huge with small wings again.
In Metroid Prime 3 he was ginormous as Meta Ridley. As Omega Ridley he was suddenly a lot smaller. Yes, he changes size in the very same game, and a Metroid game nonetheless.
In Brawl he was big, probably his best size yet since he has both big wings and body. Changes when he becomes Meta Ridley to smaller body and bigger wings.
In Other M he was suddenly ginormous again.
Furthermore, Smash Bros' is a spin-off, so size is a big whatever. It wouldn't be fun if the game couldn't give canon the big middle finger here and there, that's why we see Queen Bee and Petey Piranha playable in Mario Kart. Heck, it's the reason we see Bowser playable in spin-offs to begin with. Sometimes, canon needs to be broken in order to make way for fun, and Sakurai has done that plenty of times by now, Ridley should be no exception to that. If Ridley isn't in as a playable character, it'll be because of other reasons.
The two are hardly compareable, and Charizard is only 5' 7'' in the games, pretty accurate if you ask me, or anybody.
Not a very lucky example since Charizard has become bigger in X & Y