Overall, the content of your essay, not the spacing, should be the focus of your energy.
Be sure to pay attention to everything from the title to the style, and think twice before selecting any of these bad essay topics.
There are reasons why double-spacing is the standard for the essays you write in high school and college: double-spacing is easier to read quickly because the lines don't blur together; also, double-spacing gives your reader room to write comments on your essay (and yes, some admissions officers do put comments on essays for later reference).
Always use a standard, easily readable 12-point font.
Should the essay be single-spaced so that it fits on a page?
Should it be double-spaced so it's easier to read?
If the application provides formatting guidelines, you should obviously follow them. An applicant who can't follow directions on the application is someone who is likely to have problems following directions on college assignments. So if a school says to double space with a 12-point Times Roman font, show that you pay attention to both details and instructions.
If the application does not provide style guidelines, the bottom line is that either single- or double-spacing is probably fine.
You can't even hit the tab character to indent paragraphs.
Most important will be choosing the right essay option for your topic and writing a winning essay.