In case people don't really understand the story, a user from DeviantART was able to figure out how tall Rosalina is when she's compared to Mario, Luigi, and Princess Peach. He used the Super Mario Galaxy models while working on the size comparisons, and it appears that Rosalina is 7'7", rivaling the height of Mega Mewtwo X, being an inch taller than Ganondorf, who's 7'6", and is surprisingly taller than Dragonite, who's 7'3".
Now, following Sakurai's description, as tall as Rosalina is, there are creatures other than Bowser and Donkey Kong, who outsize her. This even includes certain Pokemon, such as Rhyperior (7'10") and Beartic (8'6"). Even Rayquaza heavily outsizes Rosalina by several feet.