The common formatting for a MANUSCRIPT is 1.5 or double-spacing with a courier type font, 1" margins all around, and actually 2" for the top (1" for the header that has your book name, your name, and the page number).
For TYPESETTING (which your publisher deals with, not you) is typically some fixed spacing per line, usually 1.25 or so. But that's their issue, not yours. They will use whatever font size and spacing is necessary to make the book fit their ideal for book size. Some books will use very small fonts and tight spacing while others will use larger type and extra spacing. It also depends on the intended reader (larger type, more space for teens, smaller type, less space for adults, or larger type for hardcover, smaller for paperback). Again, unless you plan to self-publish, its not a concern you need to worry about.
If you DO intend to self-publish, either as an e-book or your own private printing, then the best suggestion is to look at other similar books and figure out what they used. (Just type the exact same lines from a published book, measure the margins they used, then play with the font and line spacing until it looks similar.)
