Yes, re-sizing him isn't out of the realm of possibility. With Ridley, it shouldn't be an issue. So long as Ridley is reasonably bigger than Samus. Much in the same vein that Bowser is still reasonably bigger than Mario in Smash. Thing is, ever since Bowser has been given a 3-D character model, he has been quite larger than Mario in most games. The most noticeable of these occurrences is in Super Mario Sunshine, Super Mario Galaxy 2 and even Super Mario 3D World. Ridley is usually seen as quite larger than Samus. But in a few 2-D games, their size differences aren't very noticable, or at least minimal.
One other thing that Ridley and Bowser share is that in some of 2-D games they tend to be quite smaller. In Super Mario Bros 1 and Super Mario Bros 3, Bowser's and Mario's sprites are nearly the same size. In Metroid 1 this is the exact same case between Samus and Ridley. In Super Metroid, Ridley is bigger than Samus, but the difference isn't major. The difference in their sizes is similar to Mega Man X and Sigma in X4 I think. It's noticeable, but not extreme.
Bowser isn't the only character to get re-sized in order to fit. Ganondorf in Twilight Princess is twice the size of Link and Zelda in that game. I think this might also be the case in Ocarina of Time as well, but I am not 100% sure on that one. Yet in Smash, Ganondorf is only one head or so higher than Link and Zelda.
Some characters are sized up as well, Pikachu being a little over 1 foot tall in Pokemon. While being scaled up to half of Charizard's size, Charizard is normally 5.7 feet tall. Kirby is normally 8 inches tall and Olimar is supposed to be the size of a quarter.
That said, Ridley should be one of the bigger characters. It isn't really that hard to imagine him still being implemented.
Errr.....Sorry for the wall of text there guy. But I figured I'd do my best to answer your question.