Some of you may find this interesting, I might turn it into an article for the SDL but for now here's some facts, inferences and conclusions.

400x400 blank grass : 2.29 MB (Largest)
200x200 blank grass : 658 KB
100x100 blank grass : 231 KB (Standard)
15x15 blank grass : 89.2 KB (Smallest)

Terrain type has about a 10 KB difference average

100x100 covered in trees : 1.36 MB
100x100 blank grass 40 elevation : 231 KB
100x100 -24 elevation (water) : 231 KB
100x100 blank grass with one huge square of 40 elevation : 244 KB
100x100 blank grass with one huge square of -24 elevation : 238 KB
100x100 blank grass with 0 elevation : 231 KB

Inferences:

A map with one steady elevation will not change size at all. If varying elevations exist than file size will grow. If there are a lot of trees on a map file size will increase dramatically.

Conclusions:

Varying elevations will increase file size but not very dramatically so lots of different elevation isn't bad. Don't put trees on empty areas of the map, leave areas that can't be seen blank with one elevation.