One thing I'd like to point out is that Ridley being the size he is would almost certainly be the largest problem for Sakurai's team, and not that he can move freely throughout the air in the Metroid games. There are definitely ways to make Ridley have an easier time in the air, and that would be the thing that I feel Sakurai and his team would figure out quickly. The reason size the the problem here is because of how Sakurai conceptualizes it. He knows Ridley can get smaller, he's just worried about how much smaller he can make Ridley, as well as how different from the other members of the cast etc. If Ridley gets too small, for most other people, they wouldn't mind, but for Sakurai it is huge because he loves the attention to detail. If Ridley were suddenly only a little bit taller than Samus it might not look bad, but from Sakurai's perspective as the developer it is highly inaccurate to the series.
I'm not saying that Ridley is ultimately too big either, because he can definitely be resized and put into the game if Sakurai wants to, but Sakurai would have a lot of trouble working out proportions AS WELL AS trying to make him move well in the air, and that is where it becomes a problem. If Sakurai thinks that making Ridley would be too much fuss for something that wouldn't be completely accurate to itself, he's then looking at something that doesn't quite fit a professional game in his head, despite the fact he doesn't know what the outcome is. Making Ridley big enough, but still small enough would also come into animation etc., and if they want him to look the best he can that might even include redesigning him, again presenting a larger challenge than thinking of a way to make him able to move smoothly in the air and incorporating it into his design. This is probably why Ridley wasn't included in Smash 4, but is also why it is possible for him to appear in the future.
IMO there really isn't too much to discuss here regarding Ridley, which is what I think Sakurai was aiming for when he sent out that E-Mail. I think the biggest issue with that E-Mail was his stuff about characters representing themselves.