The D&D Fifth Edition Dungeon Master's Guide on p14 suggests using three map scales:
- Province - 1 mile per hex, 5 miles per inch
- Kingdom - 6 miles per hex, 30 miles per inch
- Continent - 60 miles per hex, 600 miles per inch
The DMG suggests that these scales are useful in that a full page map:
- at the province scale represents the area that can be reached from the center of the map in one day's travel, assuming clear terrain
- at the kingdom scale shows a region about the size of Great Britain or about half the size of California, and suggests that such a size is plenty of room for adventuring
- at the continent scale is useful for showing how multiple kingdom scale maps fit together
Other than these suggestions, there doesn't seem to be much that hooks these particular unit sizes and scales into the rest of the rules system.
Are these unit sizes and scales significant in that they tie into the rest of the rules system somehow?