While it is widely believed that a resume should be no longer than a single page, this is actually no longer true. There are still plenty of circumstances where a hiring manager will toss a lengthy resume into their "NO" pile. But this probably has more to do with unnecessary content as opposed to length. In other words, the resume is filled with information the hiring manager didn't need or want.
A resume can be as long as it needs to be for getting the hiring manager's attention. Yet, it shouldn't have so much information that the hiring manager can make a decision on the spot and decide to not give you the interview. There should be enough information to entice the hiring manager to want to know more. This includes clear and concise delineations of career objectives, field of specialty and skill, work history, education and training, and scope of accomplishments.
When dealing with the length of a resume, consider it a marketing tool. The resume should be filled with selling points, not exceptionally detailed recountings. There shouldn't be a single word that doesn't add value to your credentials. You want to keep it as brief as possible. Hiring managers could end up looking at hundreds of submissions for a single position. They are going to initially give each resume a cursory glance. If your resume isn't burdened with irrelevant content, you catch the hiring manager's eye and convince them to give a second, closer read.
So, let's say there are no hard and fast rules for the length of your resume. There are still guidelines you should adhere to when crafting it. For example, you should keep your resume to one page if: (i) you're applying for an entry level position and have limited experience; (ii) you have less than a decade of work experience; (iii) you've had more than one position with a single employer; and (iv) you're in the process of trying to change careers, which might make long histories of what you're done seem counterproductive to your new career goal.
You should only consider a two page resume if you have more than a decade of related experience. You may also need to expand your resume if you require space for listing and verifying technical knowledge and skill. This applies to technical and engineering backgrounds. Put critical info on the first page, toward the top. Include a career summary with key credentials.
A three page resume should only be utilized if you have a strong record of leadership accomplishments. This applies to senior management, executives, CEOs, etc. It can also be used by academics, scientists and medical professionals, or any other candidate that has to list important and relevant speaking engagements, licenses, patents, course work and publications.
Any pages after page two of the resume should be added as an addendum.
Regardless of length, you still need to take care to be as brief as possible. Bullet lists should start with most important facts and work down. Do not feel every detail has to be included. In other words, even if your resume is going to be three pages or longer, keep it as short as possible so that hiring managers can easily skim it. If they find it interesting, they will proceed to read it more carefully and call to set up that interview.