Yeah, nothing is stopping him from downsizing Ridley. That's all there is to it. He even admits size isn't always relevant. Also, the only two appearances of Ridley before Brawl was a sprite based upon his old Metroid 1 appearance, where he's almost exactly the same size as Samus', so he'd be presumed the same if he showed up, the same size as Samus in Melee(maybe a little bigger), and it's not till Brawl he gains any size differences. He's never been consistent in size, which fits exactly with what Sakurai said, that characters are not consistent to size. Nobody is saying this is a hint to Ridley specifically, but it is saying "Size isn't a relevant factor all the time".
We got no clue how he'll appear in the game(he may be a sticker/trophy at best even).
...Also, you do realize that Captain Falcon is actually near accurate to his vehicles on Mute City and Big Blue, right? His giant spaceship is that big and his regular racer is really that size. It's pretty on-scale. Brawl actually oversizes his spaceship, but only during the SSE. It's smaller on Big Blue, about the real size. Also, the characters actually are scaled to the regular racers in Big Blue as well, like they could pilot it decently.