Since Rosalina's an important matchup to learn, I figured I'd test out exactly which moves send Luma flying when it's at full health, and just how far each move actually sends it. For those that don't know, if you knock Rosalina's Luma away, it starts tumbling in the air, and only stops when it lands. If it's tumbling offstage, it will fall to its death every time. Even if it's not offstage, sending it into tumble is often useful, since it separates Luma from Rosalina, and you might have another chance to knock it away.
I used Omega Wrecking Crew to measure. The stage is divided into 12 squares.
Fsmash: 3 squares
Upsmash: 1 square, only when sweetspotted
Dsmash: 1 square
Jab finisher: almost 5 squares
Dash attack: about 3 squares. starting DA point blank will cause Kirby to only hit Luma with a couple weak hits and then pass by harmlessly, starting it from a medium-shortish distance causes the final hit to send Luma behind Kirby, which can be useful if you use it at the ledge.
Starshot: 4 squares.
Swallow: about 3 squares, the only way I can get this to hit is by jumping forward+swallowing in the air. It hits Luma behind where Kirby is going.
Stone: 2 squares
Final Cutter (full combo): 1.5 squares
Hammer (no charge): almost 4 squares
Hammer (fully charged): 10 squares
Luma Shot (brief charge): 2-2.5 squares
Luma Shot (full charge): 4 squares
Notes: Nothing else will send Luma into tumble if you haven't hit it previously. This includes uncharged Luma Shot. Dtilt kind of locks Luma in place, but after 4 Dtilts, Kirby gets pushed back a bit, so be aware of that. At higher percents, stuff like Uptilt->Bair combos on Luma, sending it a decent distance away, but it's pretty clear that killing Luma immediately is extremely important against Rosalina.
In order of how useful these are, I'd say Jab and Dash Attack are the most useful for getting rid of Luma. Luma Shot is also very good, if you have her power. Inhale->Jump forward->Swallow will cause Luma to get hit by the Swallow hitbox, but it sends it flying backwards. If you Inhale Rosalina as a ledge stand option, it might be viable to KO Luma this way by turning around and then jumping toward the stage before using Swallow.
Dash Attack and Jab are punishable if you hit Luma but Rosalina is shielding, but it's usually worth it, because Rosalina won't get a big punish on you, but KOing her Luma will be bad for her.