The map content
Some special themes can not have various scales, but only the most logical ones.
For example, population density maps cannot be larger than 1:100 000 otherwise the mapped people are not representative (commuters, day labourer, etc.).
The map purpose
The map scale must be adapted to the purpose of the map and not to the first design or aesthetic idea of the author. Here you should think about how wide will the earth area be mapped be?
The map precision
With what measuring and counting will the map be built? Here you should think about how detailed the information you display on the map will be.
With large-scale maps, the information is precisely because they are less generalized.
Large scale maps are on the whole not economic, not easy to handle, and sometimes misleading.
And, small scales make the whole map difficult to read, complicated, and sometimes are meaningless.
Which scale is selected for a given map design problem will finally depend on the map purpose and physical size.
The amount of geographical detail necessary to satisfy the purpose of the map will also act as a constraint in scale selection.