Rosalind looks masculine enough that without looking closely a person would mistake her for a man while in disguise.
The clothes make the man, as they say. The human eye can get fooled by clothing. Rosalind is a beautiful lady, but she has enough masculine features that when dressed as a man she can be mistaken for one.
Rosalind knows what it is like to be a woman, and a beautiful one, and decides that in order to be safe she should dress as a man.
Alas! what danger will it be to us,
Maids as we are, to travel forth so far?
Beauty provoketh thieves sooner than gold. (Act I, Scene 3)
Clearly, Rosalind is not afraid to talk about her own beauty, nor is she afraid to take the guise of a man and hide it. She is confident and comfortable with herself. Rosalind uses her wits and her looks to get what she wants and needs. When her father is exiled and she has to flee as well, she does not sit back and cry. She does something about it, and makes the most of a bad situation.
Rosalind realizes there is power in being a man. As a man, she can play with Orlando and Phebe and both, learning what they truly think of her alter ego Rosalind while she is Ganymede.