Ridley is not guaranteed(he definitely has a chance, though), but the arguments against him(that are commonly used) are pretty weak.
However, if people think he's better off gigantic and/or a hazard, I can respect that. Of the reasons I heard, I see their point. Ridley being small is very odd considering gameplay-wise(not his appearance in the Melee intro), he was known for being gigantic overall. Some find him being made smaller an insult to his character. I can understand that view point.
But saying he couldn't work in Smash at a smaller size? I disagree. Now, I remember somebody hacking a small-sized version of Ridley into Brawl and making him near identical to his boss moves, taking a bit of attacks from Charizard too(who was the base character used). He worked pretty well in that regard, so I don't honestly believe him being small would make him a problem to play as at all. Also, a lot of people forget that the biggest issue is his tail possibly having too large of a hitbox due to its length. As for him looking horrid, he has multiple designs anyway. If Meta-Ridley looked good yet small in the Melee intro, they can certainly do it here too. I'm okay with him not being nearly as skinny or a bit more fattened up if it means he's playable.