Thilo has created a new version of his handy scale tool:
http://tile.opengeofiction.net/util/map_scale.html?map=13/20.73/86.72&map2=x/38.89/-77.04&pxpt=0.277
It's a split screen with the OGF world on one side and the real world on the other, shown at the same scale. Each side can be moved around independently, so you can compare anywhere in OGF with anywhere in the real world. By switching layers on the right, you can also compare any two places in OGF. Comparisons can be shared with other mappers by copy/pasting the URL.
As in the previous version, zooming by mouse wheel or double-click is disabled, but there is a zoom control (& measuring tool) in the upper left corner. You can also set the zoom by entering a value into the "Zoom" or "Scale" input fields and hitting "Enter." If you want the number in the "Scale" field to be accurate, the "pxpt=_____" part of the URL must be adjusted for the pixel pitch of your monitor.