from the help file:
Max level size BSP maps have a size limit that can be set here to up to 250000 units. Simple (non-BSP) maps don't have a size limit. Higher maximum level sizes can produce more warning messages with thin blocks and geometry problems in BSP levels. The z buffers of most 3D cards can not deal with huge distances unless camera.clip_near is set to a relatively high value. Use this option with caution.

A map "unit" could be used to represent just about any unit of measure within reason. I think a default would be meters but I suppose it could also be km if you want to scale everything to match. Of course if you scale too much you could loose some detail in the terrain.

As for the scripts calling each other, I'm pretty sure the answer is yes but I don't know exactly what you have in mind.