In the body of an email, rather than as an attachment, format it like you would any email: single-spaced, with an extra space between paragraphs. I don't have a link, but I know I've seen agents say to do that, b/c it's almost impossible to double-space emails, and there's no guarantee it would arrive in that format.
For the original question, I'd choose the first X pages, chosen from the double-spaced version. Remember that the agent won't necessarily read all X pages, unless you've captured her interest, and in that case, it doesn't matter whether you've got X pages of half of X pages. In other words: more isn't necessarily better. Usually, in my experience, if the first couple pages are compelling, then the next pages will be equally compelling, so the agent will ask for the rest; and if the first couple pages are weak, then the rest of the pages will be equally weak, and therefore whether the agent read 2 pages or 50, she won't ask for the rest.
JD