One page is ridiculous
Putting 20 years of experience on one page is ridiculous, particularly in this era of short-term contracting gigs. I'd have to use 4 pt. type to fit just my previous employers on one page. And bulleted lists make any given text take up twice as much room on a page -- a two-page resume becomes a four-page resume.
Advice like "fit it on one page" sounds great, but it doesn't reflect the real world. The days of working for the same employer for 20 years are long since gone.