Most platforms, like FoD Yoshi's Story and BF, are his beneficial stages.
Why?
1.He has a variety of moves that can penetrate through the platform. Some prime examples are neutral B, Uair, Nair, *Fsmash (Most effective on FoD though) and Fair. This time around, some of these moves can actually KILL!
2.He moves more swiftly and safely around a platform stage than that of a flat stage.Thankfully, there are plenty of platform stages to choose from.
Try to avoid stages like Skyworld. Yea, it's a platform stage but this particular stage has more disadvantages than advantages:
-you can fall through the cloudy platforms
-recovering to the stage is more challenging since the platform has a slanted edge and similar to that of Lylat Cruise (Minus the tilting)
-there are better stages to choose from
My overall opinion: Because of Roy's great kill moves and swift movement, He's great in most platform stages. You have a better advantage on the obvious small platform stages like FoD, Yoshi's Story and BF. But recovery is much easier when it's not a stage like Lylat due to his lack of recovery and tilting back and forth.
Oh yea keep in mind: Character you're facing matters! You have to know the difference between facing a floaty character, characters with a bit more weight and then your infamous space animals.. know what stages work on who.
Style matters! For example some players are great at edge guarding while others aren't as much. So even the bigger stages like Dreamland can be a great platform stage -- It becomes more of a preference.
I'm sure I left out stuff but hope this helps.