I'm not really sure about what specific arguments you're talking about,so I'll just take a jab at a couple of different ones.
If it's the size argument then there's over a decade worth of counterargumenting for that, just from Bowser to Olimar to Nintendo never having any problems resizing their characters if they need to, and finally the one I feel that is most important, the fact that Ridley's size isn't an important part of neither his character or appeal. Ridley is to Samus just like what Bowser is to Mario, he's not about being big or scary, he symbolizes an iconic and everlasting rivalry, it's even acknowledged in Melee's intro (yes, I know it's a common example.) Samus could be shooting at any Metroid enemy, but of course it has to be Ridley, he's her enemy. The size argument is generally outdated and has been beaten to death, so I dearly hope that isn't what you're talking about, because it's the most worn out cliché argument, and there's a reason it's become a running joke in the Smash Community.
Proportions isn't a problem either, it wouldn't be the first time proportions are twisted in the characters' favor in Smash Bros.
Hitboxes aren't a problem either, they simply just need to do exactly what they did with Charizard, have the wings and tail face outwards towards the camera so they won't be part of a hitbox.
In general there's no problem with his model or design, while he isn't an easy character to make he's certainly not impossible, and would be far from the most challenging character to make that they've done (that award goes to PT.)
About the boss "issue". There's no Subspace-like Story Mode this time around, and Smash Bros is a Nintendo spin-off series for pete's sake, they can mold and shape any character just like Mario Kart can between the original Mario franchise. Just like Bowser changes size between the canon Mario games, Mario Kart and Mario Party, so can Ridley change size between Metroid, Brawl and SSB4. In Smash Bros there's no canon facts, and the fact that there won't be a over-arching storyline just even further proves that. A change in size shouldn't surprise anyone, and if anything, a little surprise certainly shouldn't hinder a character's inclusion, aye?
I certainly acknowledge that Ridley isn't a lock in any way, I'm a bit concerned about him myself, but he's certainly one of the big contestants this time around. Before he had competition from characters which were admittedly more important than him, like Wario, Diddy Kong, King Dedede, you get the picture. This time around he's one of the last widely recognized Nintendo All-Stars, albeit more popular over here than in Japan for sure, and there's certainly nothing that makes him impossible. However, the fact that he is a hard character to pull off is one of the things that might be pulling his chances just a bit, combined with Japan's not so big love (though I don't believe they have any dislike for him either). However, Sakurai does look outside of Japan, and as mentioned he has pulled off whackier and harder concepts than Ridley before.
Plus, Metroid is a huge franchise, it was already silly that it had only one rep in Brawl, this time around it'd be a slap in the face to us Metroid fans. There's no other Metroid contestant for a playable character spot, nobody else even has decent support anymore, no other contestant has left such a big impact and is as important to the franchise as Ridley. I'd say he'd certainly be well worth the effort, if not for his popularity, then for his importance, and I'm sure Sakurai is smart enough to acknowledge that.
Also, not because I want to be mean or cruel or anything, but don't really expect to be met with a nice and warm welcome when you're dumping into a support thread for Ridley and then stating how he's not within the realms of possibility. It's really self explanatory why you aren't met with happy faces and open arms. Do it with any other support thread and you're most likely gonna be bitten too (especially ones lead by Shortie and Golden.)
That's my take on some of the things, do feel free to bring anything else up, might answer them tommorow, it's getting late here in Denmark, so I'm not going to try and answer anything else in a sleepy state.