Welcome, Akubra. Map content in the very first post - that's worth rep right there. And then some interesting geography and apt questions...

If you put a map in an equal-area projection you can use a simple trick in several graphics packages to get areas. I think serif PhotoPlus, Adobe Photoshop, and the Gimp can all tell you distribution of #pixels of this color or that-- set it to two color and you have an easy calculation for percentage land. Simple geometry gets you the overall globe surface, unless yours is oblate like Earth is instead of spherical . Then color one continent, lake, nation, whatever at a time, and you can get its size in pixels, with a short leap to size in square kilometers or acres or what-have-you. Make sense?