Go with business sized envelopes. They are set up so that you place two folds in them (dividing the long side of the paper in thirds). They're the most commonly used size of envelope in the business world. Manilla envelopes are only for things you really don't want to bend, such as an 8x10 picture or an inch thick stack of papers.
Likewise, do not choose the small envelopes that you have to fold your paper up to 1/6th the size to get it in.