Regarding the map I drew a while ago, it would be nice if I could just find out what the area (in square units/sq km) of my island is. Does Inkscape or any other software have a tool to do that?

DCJC

AMDG

... -> make island one color, ocean another, export to bitmap, remember km to px conversion factor. import in Gimp, Tools-> Histogram, pick the color of the island, read pixel-percentage and calculate back to km^2.... Possibly very tedious...

Take a look at the Measure Path effect under Effects / Visualize Path. It allows you to specify things like units, the ratio of drawing to real life, etc. You can probably get it to do what you want with some playing around.