Q. What margins should I use?
A. One-inch margins all around is a typical default setting for manuscripts, whether you’re using 8.5 × 11–inch or A4 paper, except for any headers and footers, which are typically half an inch from the edge of the page. One inch is equivalent to 2.54 centimeters or 6 picas; half an inch would be 1.27 cm or 3 picas. That’s what a US English installation of MS Word shows when the measurement options under File > Options > Advanced > Display are changed from inches to centimeters or picas, respectively.
These default settings are fine to use in most cases. But if you’re writing for a publisher, consult your publisher’s guidelines (and see “How to Format a Novel for Submission” at CMOS Shop Talk). If you’re writing a dissertation or thesis or other type of paper for school, consult the latest edition of Kate L. Turabian’s Manual for Writers of Research Papers, Theses, and Dissertations (a.k.a. the Turabian manual).