I also usually take the side of art and topology, rather than necessarily topography.

For features ... average sizes and so forth ... I tend to rely on experience and places I've visited. If I imagine I need a river in the desert I might be reminded of the Nile, I'll go on Google Earth and measure it's width and a given location and so forth, to get an idea of the size. If I want mountains, I've been to different types and sizes of mountain, I go check out some typical mountains and measure them again. But ... this is more relevant for city maps, where I find some scale is more important than on a more symbolic, large scale map.