Currently only one size can be set for both panel and its content. What I'd like to see is being able to have two size settings, one for panel itself and the other for its content (i.e. dock icons, launchers, etc..) My goal is to have small panel and bigger icons reaching out of the panel, while the "apparent" panel size reported to the WM is the size of the panel and not the conent so that when a window gets maximized it won't stop the border on the content but rather the panel itself (i.e. the panel content will overlap the window if it is bigger than the panel). I'm not into any fancy effects here, I think this could, with enhanced transparency settings (see my other enhancement request), improve usability.
The 4.8 panel will have a setting to disable struts (ie the code that tells the window manager not to overlap the panel. 2 size is too user specific (and you could get the same result with disabled struts and workspace margins in xfwm4).