Here’s a map view intended to help with mapping objects at the right size:

http://tile.opengeofiction.net/util/map_scale.html?pxpt=0.277

You can view the map either at a fixed scale or at fixed zoom, depending on which of the input fields is selected. For the scale to be correct, the pxpt URL parameter (the pixel pitch, in millimeters) needs to be adjusted for your monitor.

Note that zooming by mouse wheel or double-click is disabled, but you can set the zoom by entering a value into the “Zoom” input field and hitting “Enter”.

You can also select the “OpenStreetMap” or “OpenTopoMap” layer for real-world comparisons, and below the zoom control in the upper left corner, there’s also a measuring tool included.