I honestly don't think Sakurai and co. have any strict guidelines as to how drastically they can scale characters, I think we as fans overanalyze the situation like crazy, it's probably more a case of whether a character still feels right when scaled up or down, which is the case with Olimar, Bowser, Ganondorf and the Pokémon. A lot of Sakurai's character planning seems to come down to nailing the right feel, there's a quote somewhere out there where he says that he'll only try and put a character into Smash if he can see exactly how it functions inside his head, if he can't imagine it then he won't even entertain the idea (as was the case when Pac-Man was suggested to him for Brawl). When you combine that with what he was saying about Ridley (too big, can't fly freely) then I think it's pretty clear Sakurai doesn't believe he can make Ridley still feel like Ridley with the restrictions that being a playable character comes with, it's not necessarily a case of Sakurai thinking it's too drastic to scale someone of Ridley's size down as it's him thinking that being big is a core part of Ridley's character.
I don't agree that size is all that important to Ridley of course, but I can understand and respect that line of thinking much more than if they had set a limit as to how big a character can be before being taken into consideration, like some people are suggesting is the case. That philosophy is why every single character in Smash feels faithful to the source material (with the obvious exception of Ganondorf, which was a bit of a special case to begin with), the only downside to it is that it's very subjective to Sakurai and therefore every potential character is at the mercy of his own personal opinion, something Ridley has unfortunately gotten on the wrong side of.