I found this image on Gamefaqs a while ago, but I forgot who did it.
Ridley is compact, he can tuck all his limbs closer to his body and reduce his idle size and stretch out when attacking giving him deceptive reach, his wings and tail can always curl around him when he's not attacking with them. If you remember in Metroid Fusion, Ridley's frozen corpse is half the size of Ridley-X when you battle him and in the cutscene in Zero Mission of him perched he is also smaller and compact. So long as they don't make the mistake of making him slow, and heavy it should be ok. Ridley should be lightweight but fast and strong like he is in Prime. He has no shortage of moves to choose from, teeth, claws, wings, firey breath, a sharp tail and projectile weaponry.
If they wanted to use him as a boss there's no need to not make him a playable character as well, they can always make a larger, stronger Ridley, ie Meta-Ridley or Ridley-X for that which could plausibly be cloned by the space pirates or something. Ridley is inconsistent between the Metroid games, his height and colour frequently change and Brawl isn't canon anyway.
Just using Samus to represent Metroid isn't a great idea and Ridley is the most useable non-generic villain in the series. Mother Brain would make a great boss battle only stage, why constrict Ridley to just a boss character?
I'm still not fond of the mechanics of her final smash and selecting ZSS either.