Double-clicking a panel in Graphic.ly Flow should zoom in the panel to 1:1 (100%). Currently a double-click returns to the spread, which to me seems counter-intuitive. Usually a double-click is used for confirmation or, in some cases, maximization/magnification (like double-clicking a title bar in Windows). For another example, think of how double-tapping the display on the iPhone version of Safari toggles between viewing a block of text and viewing the entire page.
I would at least suggest a quick way to zoom to 100%. Using the mouse wheel is actually quite cumbersome and it's difficult to zoom in without going way too close. The double-click could be a toggle between 100% and then whatever the default panel view is (this appears to be 100% height of the display). If you would like further clarification I'd be happy to provide details.