Rosalina has amazing range with her hitboxes, but it takes some time to understand them.
For example, when Luma is attached to her, the two main hitboxes are right in front of Rosalina and the area in front of Luma. But there's a spot in between Rosalina and Luma in which her Smash won't hit nearly as hard.
But if you manage to hit with Luma, the base knock back is incredible. Luma's forward smash consistently KO's opponents around 60% if you hit them at the edge of the stage, no exaggeration. I'm not even talking about against characters with bad jumps that can't recover to the stage. I'm talking about knocking them clear off the map. Hitting with a charged Luma forward smash in the sweet spot is basically a Warlock Punch.
Her up-smash and espeically her down-smash cover a lot of range, when Luma is with you. But her up-smash won't hit at all if your opponent is behind both you and Luma, so be carefuly of that.
Other than that, I'm not really sure what to say. Rosalina and Luma have amazing hitboxes, once you understand how they work.