[Marianne's] form, though not so correct as her sister's, in having the advantage of height, was more striking;

I can only make sense of that grammar by reading "in ... height" as an adverbial phrase referring to "was". Hence Marianne is the taller, and there's no reason to suppose that height had anything to do with correctness of form.

Elsewhere, JA quite often refers to plumpness and bloom as positive aspects of beauty. This fits with an old fundamental view, that beauty equates to the visible signs of good health. Thinness and pallor aren't a good look when, say, tuberculosis is a major threat.