Thanks Dain and Arsheesh!

I have spend the last 2h of thinking and reading to come to a conclusion that I will have a Scale of 10Km per mm on the map, to remind the map will be 1200x900mm. Important was for me to have sources about how far a REALISTIC medival Rider can do, or a merchant etc. to have the distances in check. In the end the Main Empire will span about 5000 x 4000 km, with Letter Pigeons (1000km a day, with 4 days Delay of information at optimum and 10 Days at worst case) which is more on a mathematic way very important to my way of thinking about maps. its all math and informationstructure....