People don't seem to understand that Mario is simply a fantasy series that inherently plays with scaling and does it all of the time. Metroid is not. Bowser has one scale when he is a character (Smash, Mario and Sonic, Mario Party, Mario Kart, sports games, etc.) and a varying scale for when he is a boss. You can fudge a fantasy series like Mario and you can do it interseries with some others, but it causes problems when done significantly intraseries.
Anyway, Sakurai has already shown us that he knows Ridley's scale in Brawl. Super Metroid size or bust.
Finally, someone with a brain whose head is down to earth rather than in the clouds. I'm going to attempt to put this to rest once and for all.
As anyone, I would love for Ridley to be a newcomer
if it made any sense at all. Unfortunately, there are too many factors that Ridley has which would pose a serious canonical conundrum. Ridley is too big, yes- it has been said before. Allow me to add new merit to this argument. The height comparisons on the OP scale Ridley to be smaller than the Pokemon Groudon and Zekrom, which is completely not to scale. Ridley, in the Metroid games, would certainly be comparably larger than each of them. Sure, Bowser is a large character, but regular Ridley's height is closer to Giga Bowser than Bowser. Bowser's height in Melee and Brawl is true to most Mario games. Ridley's, should he get in, will not be true to his games. So right there, Sakurai and the whole SSB4 team would be taking a departure from Nintendo's beloved Metroid series in order to appease eager fans.
But wait, it gets way worse!
Ridley, being a dragon-esque creature, can fly perpetually. Not only that, but he can fly so fast that if he flew overhead a sprinting Sonic traveling in the same direction, Sonic would stop and ask what the hell that was. For obvious reasons which need not be discussed, these cannot be attributes of any character in a SSB game. They would have to limit his flight, rendering Ridley quite oddly nerfed relative to his flying prowess in Metroid.
No, it's not over...
Ridley possesses the ability to fire out gigantic plasma fireballs in rapid succession, the like of which make Bowser roar in jealousy, especially since these fireballs can easily be larger than Bowser himself. They would have to be limited in size, proportional to Ridley's newfound height should he become a newcomer.
Ridley has many forms to boot. Which form would be incorporated into the game? Regular form? Meta? Mecha? Neo? Robot? Would every form be represented into Ridley's gameplay? How would that work and translate to SSB4?
As the commenter whom I am replying to put it, Sakurai already secured Ridley's place in SSB as a boss. A fitting choice as Ridley is a larger-than-life character who deserves to be represented accurately in whichever game he might appear in. That said, I am all for Ridley remaining a boss fight in SSB4 as well as being a new assist trophy.
So, before all you Ridley fans jump on the bandwagon, think about all these dilemmas. Jumping the gun and saying "to hell with Metroid canon" might disenfranchise some diehard Metroid fans and critics, not to mention. I surmise it is too astronomically obscure to add Ridley as a newcomer in SSB4 for the reasons aforementioned, and the SSB4 team already understand this which is why he made his debut as a boss in SSBB. The only way each of these issues could be resolved would be if an adolescent form of Ridley was introduced and used in SSB4. Clearly, this form should as similar to Ridley as possible, meaning it should be greatly older than Little Birdie that Samus discovers in Other M. Such an innovation would quite possibly reconcile these apparent problems facing a prospective newcomer in Ridley. However, as this has not yet been introduced, it is unlucky to find this younger form of Ridley in the next SSB game.
I'm sorry to be a buzzkill, but I will eat my words if Ridley is in fact confirmed as a newcomer. If not, I don't want to say I told you so, but...
I told you so.