I started with this picture of his tail shadow. I reasoned that this tail thickness is likely going to be the thickness of the real Ridley's tail, owing to the consistent shape and thickness across the stage and the overall tendency of shadows to become bigger, not smaller.
I then measured out that four Ridley tails was the sufficient number in order to equate the thickness of this brick on Pyrosphere.
I took an Other M Ridley model (because that was the most likely model and I wanted the hugest Ridley possible), bundled four tails, and scaled them appropriately on an top-down view of the Pyrosphere.
(Yes, I verified that the Other M Pyrosphere was roughly the same as the Smash Bros Pyrosphere, the only primary difference being that the Smash version was more octagonal.)
Anyhow, I lined up the two Pyrospheres, and...
There we have it! Ridley's Smash Bros. size.
And a quick size comparison with Samus for added effect.