I'm writing a fantasy novel, and I need to get the geography right?

[Jason]

Since I have the story and all mostly planned out for my story, I turned my attention to geography. I made a map of the area in which the novel takes place. After doing much of the first draft of the map, I measured the average town to be about 15 miles across diagonally (so it doesn't matter it is square or not). Is that too big? I know it's my story and I can make it any size I want, but since I don't intend to have these towns filled with tons of people. I haven't decided an average as far as population yet. I just know I don't want too many.