You could e.g. place some dummy floor on the map that is the exact same size as your screenshot and than you can ingame check its xy coordinates and size and compare it with the pixelsize of the whole map.

Other ways for minimap:
- dynamic through VIEW
- screenshot from terrain and 2d icons for entities (like red dots for enemies), I believe this is how some rts games do it (e.g. use panels for that).
- iirc a shader one (probably by txesmi)