While Ridley's size definitely lends itself to his cinematic exposure, it doesn't directly harm his chances of getting in. We've gone over this before.
Not only does he change size throughout Metroid, but he has changed size over the course of Smash Bros. In the Melee intro, he is roughly the same height as Samus, but in Brawl, he is clearly much larger. Really, Ridley is a fictional character so it doesn't matter what they do.
However, if this were a fighting game called "President Evil", and they planned to put Abraham Lincoln vampire slayer in, making him shorter to fit with the cast would be silly. That would be because that literally would make no sense because he is known historically as a tall man. A similar case to this would be making William Howard Taft or Grover Cleveland skinny.
To get away from that random tangent, my point about Ridley's size is that really, it doesn't matter. So many other factors are put into the game when creating a character. Do we really think that something like the size of a character would stop Sakurai, the guy who includes such wacky things in Smash Bros., from fixing this issue with a mere downsizing? Ridley doesn't have feelings. If he DID have feelings, he would probably prefer to be downsized if that was the only way he could get his hands on Samus, right?
...although that President Evil idea sounds pretty awesome.
