While this is a valid thought, what needs to be brought into consideration here is the fact that perhaps the game directors don't want to "stay as faithful as possible". A good example of this would be Olimar, who in his games, would normally be small enough to fit in one's hand; however, in SSBB, Sakurai and his team made him a little shorter than Mario's height. While Mario, of course, is shorter than the average male human (this size which could be represented say by Link), he is by no means microscopic. The fact that Sakurai and his team decided not only to enlarge Olimar, but make him close to Mario's size (approximately 3-5 ft in height) shows they are willing to change characters a large amount so that they can fit in the game.
Of course, Ridley is a different problem altogether in the sense that he is not being enlarged, but rather downsized, and a character his size could indeed be "tough" to make smaller due to the way his model is (Lanky, but large). However, this goes back to the fact that Sakurai and his team might not want to "stay as faithful as possible." We know of course, that in the past, Sakurai has commented on Ridley saying that it would be "difficult" or "impossible", but as is the case with The Villager, it is certainly plausible that he may reverse his stand on these subjects (as he originally didn't think an AC representative would work within the Smash Universe).
Now, back on the subject of staying faithful to a character's origins; I've already mentioned the Olimar example, but I would like to make a point on what you said regarding Ridley's size in Melee's intro (wherein he was maybe 1/2 - 1 ft taller than Samus). As you mentioned, Ridley is noticeably smaller than his size in the Metroid games during this intro, however, you say that size wasn't an issue because it was just an intro. But, if a character such as Ridley can be downsized to that point in the intro, what would be the problem with having Ridley in the game at that size or maybe a little smaller? What I'm trying to say is, if there is no problem with having him smaller in the intro, why is there a problem with having him smaller as playable in the actual game? (I don't mean to sound like I'm attacking your opinions here, by the way). Yes, this would technically not be with respect to Ridley's origins in terms of his original size, but again, is it really that big of an issue?, other characters have been resized, and while certainly their situations are different from Ridley's, I still don't see why a resize of Ridley is still considered impossible by some because a good number of these characters in the game (such as Olimar) can only exist well in the game if they are resized to a point that is different from their original size. Since, other characters have already been resized so that they can work in game, I ask the question one more time, Why not Ridley?
PS: Sorry for long post and if I seem like I'm rambling, I express my thoughts best in streams of consciousness writings like what is above. Thank you for taking the time to read this absurdly long post on Ridley.