First of all, Metroid needs more characters. It's one of the biggest franchises of Nintendo, and unlike many other franchises that are much less important, Metroid still is stuck with one and a half character (in terms of roster size at least).
However, since there's really not many notable names to pull from a series that mostly focuses on a sole bounty hunter crawling through the depths of a few planets, there's not really many solid choices to make from the good character side, so we'd have to look at the villains. Ridley is the best choice for a few reasons.
For one, he's in every single iteration of Metroid, save like 3 games - Metroid 2, Metroid Prime 2 and Metroid Prime Hunters. Thus he has more appearances than any other villain of the series, except the namesake of the series itself (which in fact would not make a good character, since all Metroids do is latch onto people and draining their life).
Second, he is considered to be Samus' arch-nemesis. He was the one responsible for the destruction of her homeworld and the death of her friends and family. And he is the adversary who just refuses to die; time and time again, Samus has to face Ridley and defeat him. If anybody could be considered being the main recurring bad guy of the Metroid series, it's Ridley.
Third, while he generally is depicted as being bigger than Samus, he has also been depicted as being just a little bigger - most notably in the intro of Melee, where he is roughly the same size as Samus is. Since Smash grows or shrinks characters at its own leisure (being not canon and all), there is nothing speaking against the game changing Ridley's size to accommodate the mechanics. And, after all, there's several characters that hulk over others, such as Bowser or Ganondorf, so you could still make him bigger than the majority of the cast.
Fourth, he has great potential for a moveset - we have yet to see a more feral, flying creature like Ridley in Smash.
And finally, he has been making non-playable cameos in the Smash series since its very first iteration.