I don't know if geology played that much of a huge role in drawing the landscape. Mostly it was just a large amount of manual work drawing the shapes of the land. Though I did often use various Earthly coastlines as inspiration when doing the coastlines in detail. But the tectonic stuff really came into play only when the coastlines were finished. Glad you like how it turned out, though!

The land area can be calculated by using GProjector to switch to a Mollweide projection (or some other Equal area projection). The map should be a simple black-and-white map depicting the landmasses. In GProjector remember to leave the graticule and the border out, and the background needs to be a different color than black or white. This is done because you essentially want a 3-color map (background, water and land each in separate colour). Then just save your map in some format (png works well for me), with the best possible resolution (6800x3400 has been the "practical limit" for my gprojector before the program freezes).

Next up, import your fresh map into an image manipulation program (I use Gimp), and use the "Select by colour" tool to select all the landmass. Now there should be some way to show you the amount of pixels within your selection. In Gimp you can do this by selecting Windows->Dockable Dialogues->Histogram, and it will show you the pixel count. If you use some other program, you can probably find a way to do this by googling. Anyway, once you have the pixel count for land, write it down and do the same thing for water. Then you know the total pixel count, and the land pixel count, and can calculate the percentage.