Look Entei, Ridley is constantly depicted(even in ssbb) as a GIANT space dragon. Other characters have been scaled to be made playable, but not to this extent. That's not to say it's impossible, but all the optimism and sarcasm in the world won't suddenly resolve Ridley's issue.
All the pessimism in the world won't keep Ridley from being playable either. I simply don't see the problem, canon matters nothing in this situation, especially when developers like Retro Studios don't even care about his size themselves, changing him from his biggest size to his smallest size since the original Metroid in the very same game.
I mean, just look at this little thing I put together:
I'm well aware that there's some distance between Omega Ridley and Samus in that picture, but it doesn't take a genious to see that Samus can't fit in his mouth. If you look at other points in the cinematic, you can see that Ridley can probably fit Samus' arm cannon in his mouth at max.
This is in the very same game.
So tell me, if not even the Metroid games give a **** about making Ridley's size consistent, why in the name of all living things should a spin-off game do so? I want an answer to this, or you won't be able to seriously argue any further. And yes, this is the second newest installment in the franchise, so it's not like it's outdated either.
Point is, Ridley is constantly being resized to fit into the situations he's being put into. In the Meta Ridley fight in MP3 he needs to be enormous in order to make it a very hectic tunnel fight, he needs to pretty much take up all the room in that tunnel to function properly. However, later in the game, as Omega Ridley, it's supposed to be more of an even battle on even ground, and thus he is scaled severely down in order to fit that scenario.
If Ridley is a character in Smash Bros, then he will get scaled down to possibly a smaller size than ever so that he can fit that scenario, thus it doesn't break any rules that have been set. He simply hasn't had a reason to be playable character small yet, but now he has a reason to, and I don't see what the fuzz is about. Spin-offs are here to break the rules, but in this case, there aren't even any rules to break.