For essays, some schools specify line spacing, and some don't. If there are no guidelines offered, I'd shoot for 1.5x spacing. Single spacing makes it too rough on an adcom reading your essay, and 2x spacing just seems to "high school" for me.
Resume length is generally 1 page anywhere in the US (with exceptions, but most applicants should fit it into 1 page). Try not to fudge the margins too much, as this could backfire when the adcom goes to print out your resume, and either 1) parts of your resume are chopped off, or 2) they have to go to great lengths just to get your resume to print properly. You don't want to be "that" guy who causes unnecessary trouble.
For sample resumes from current students at various b-schools, visit this thread: best-resume-cv-format-template-tips-85627.html
. Note: you don't have to change your resume every time you're applying to a different school; rather, just find one that you think looks the best, and format your resume accordingly.
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