make the distribution of terrain types in my map realistic

[Riptide]

I have written a fantasy novel and have created a map for it. However, I have little idea of what the distribution of different terrains should be like at its scale. For example, how many pixels the individual forests within farmland should take up, how large the gaps between farm pixels in the desert should be, and things like that. (Each pixel is about 3 km across.) The world has roughly 15th century European technology with no magic. The climate changes from a desert one in the far south to a temperate one in the far north.

I am fully aware that the massive areas of forest are unrealistic; I originally thought up the map when I was 13. I don't see the point of removing them until I know exactly what to replace them with.

Any other feedback is welcome.