If the bike pictured really is from ~2006, and if it has 700c wheels then I'd bet a dollar that it is the size they called a 58cm. This Cyclofiend page has the historic Atlantis Geometry info.
http://www.cyclofiend.com/rbw/geometry.html
Measure from the center of the cranks, along the seat tube to where the top of the top tube intersects the seat tube. That should be damn close to 58cm. If it's not close to 58cm then decide what size on the chart is closer. To confirm it's a 58, match some of the other numbers from the geometry chart.
Bill Lindsay