How long should a resume be? The short answer is: long enough.
Standards have changed. The old rule about never going over one page has been thrown out, and for good reason.
First, that rule often confined applicants to merely listing their duties. Modern employers are more interested in what applicants have achieved. Every shoe salesperson sells shoes. But you stand out from the pack if you were among the top five salespeople or if you specialized in athletic shoes or if you sold in one of the largest shoe stores in the country or if you took a course to improve your sales technique.
A longer resume lets you describe your skills and how your old employers benefited from those skills.
Second, people are moving around more rather than staying in one job at one company for their entire careers. A resume should emphasize past jobs that directly relate to the job an applicant wants now; and in general it should concentrate on recent jobs rather than the distant past. But that still leaves a lot of employment history for most people: too much to fit on one page.
How long should a resume be? It is unacceptable to fill your resume with all fluff and no content. But so long as your career justifies it, two (or even three) pages are fine. As a professional resume writer, I can make sure that every page counts.