One way to answer this question is to compare the outward affect of the characters, and approach the question as a director or actor might. The two characters are sisters, but it is necessary to have both physical and personality traits that distinguish them.
Rosalind dresses as a boy (Ganymede) but Celia remains in women's clothes, though dressed in rustic mode as a shepherdess, to maintain their disguise as forest dwellers. Rosalind also mentions being "more than common tall" which suggests Celia is shorter; this also suggests a slightly more masculine demeanor in Rosalind, who, as Ganymede, speaks boldly to men and has no trouble convincing others that she is a man.
Celia teases Rosalind/Ganymede when she mentions seeing Orlando, and when Rosalind, regressing to an excited, feminine flurry of emotional excitement, bombards her with questions, she replies: "You must borrow me Gargantua's mouth first. 'Tis a word too great for any mouth of this age's size." This again suggests a physical difference in size between the two, and suggests a casting decision to be made with Rosalind being played by a taller actress.
When Rosalind speaks to Orlando about her sister's love for his brother, she is suggesting parallel connections among the four: "Your brother and my sister no sooner met but they looked, no sooner looked, but they loved," etc. and in this way also emphasizes the immediate attraction between Rosalind and Orlando (and Orlando's more latent "attraction" to Ganymede, portrayed as male bonding).