Sonic's air speed is the 6th fastest, with not so bad acce and decce too (From what I remembered).
Remember, his recovery is rather limited to Spring Jump. Homing Attack isn't really good since if it fails to home in, it sends Sonic downwards, which basically means death unless he still has his double jump. Spring Jump itself is very vertical. Snipe his double jump and hit him if he is still not dead, should finish him. Or D-air spike him.
Spin Dash are annoying, but one thing Rosalinas should watch for is Spin Charge. Yes, you all probably are going like 'Those moves are exactly the same' but they are not. Spin Charge is faster than Spin Dash, and hits multiple times, and still combos into aerials (Albeit harder). This can catch people off-guard, so be wary.
His sheer speed (Dashing speed is over 3.5.... Falcon's only 2.3. The 'fastest thing alive' thing is real) means no mistake in your part go unpunished. I'd be wary for his U-smash, it's unreasonably strong for him, and Rosalina's vertical survivability is really bad. Oh, and Spin Dash -> U-air -> Spring Jump -> U-air, the ultimate noob friendly yet very rewarding combo. Getting hit by Spin Dash can mean a stock, so don't risk it if you can't.
Otherwise, he has fairly short range. Disjointed attacks should work against him. For juggling, remember he can cancel his D-air landing lag by going high enough, and his D-air falls really fast. Juggling him won't be easy if you can't catch the darn hedgehog himself. Though, if you ARE already in the air... then yes, snipe his double jump and smash him with U-airs.
That's all I can comment.