His passing is based on hip redirection and movement. I think it would work fine on a big guy. I don't think body type has anything to do with it, at least not the body type of the guard player (it matters more for the passer, as huge guys generally aren't that mobile from a deep squat like Rafa).
His guard game is also fine for the open class. The Mendes bros are fanatical about all guard positions they play having great distance control, that's a huge part of playing against bigger people. You can see videos on their website of him rolling with much bigger, very high level guys and his guard works great.
The kinds of things I worry about for little guys vs. big guys are if the small guy's game depends on having approximately equal strength, or being able to use your weight to smash, basically if the smaller person has a game built on bullying the other guy. Celsinho, for example, I wouldn't expect to do well in open weight divisions because he loves to smash and grind people his own size and I doubt that would translate well (Augusto Mendes is another guy who I doubt would do great against much bigger elite competition). Rafa's game really isn't like that. On top his game is about timing and movement, on bottom it's about distance control. That's a really good recipe for dealing effectively with larger people. Even though his game was very different, Marcelo had the same features in his game.