I've been in HR for 7 years but anyone who has done a good resume will tell you it's much more difficult to do a one page resume than a three page resume. It's an art to put together your life's work down on a page of paper. It doesn't change the fact that administrative resumes should be one page exactly. Not half a page and certainly not a page and a half. Anything more or less and you look unorganized and like you might be high maintenance.
Even the best recruiters and managers won't spend more than 30 sec on a resumes so making sure your critical skills and accomplishments get noticed is key. You can change the font, spacing and margins to get more room but make sure it still looks organized and visually pleasing.
Most people should have a one page resume. There are exceptions, engineers, scientists, high level marketing folks, C-level and high level VP folks can all have resumes longer than a page. Most of us have to stick to one page, me included.