The resolution and scales are up to you. You may start with a 4K x 2K pixel world map which is in a Mercator kind of projection so that the scale at the equator is the circumference of your planet divided by 4K. So a 40960 km circumference planet has 10 km per pixel at the equator. But owing to the projection, the scale of the map as you approach the poles is going to change.
But lets say that you want to map a small country or island at the equator area. You have an island that is 41 pixels across on your world map. So that is 410 km across island. You can now use another 4K x 4K map of that island. It is now 410,000 m across divided by 4096 which is about 100m per pixel. On that island you have a city. That is likely to be in the region of 10 to 100 pixels or so depending on how large the city is but lets say its 20. If you made a city map of 4K x 4K you now have 2000 m divided by 4096 which is about 0.5 m per pixel.
If you take the city map and print it out so that it now has a physical size instead of a pixel size and lets say you print it large like 1m x 1m then now you have a city printed at 1m across representing a size of 2000 m. So the scale of the map is 1:2000. Only when you print the map to a physical size can you say what its scale is as a real ratio. You could measure how large your monitor is and work out its pixel resolution and make that calculation for the times when you show the image without image scaling but with just an image you can only generally say what its m per pixel is.
Going back to the world map and projection. If your using a Mercator projection then as you head towards the poles then the map gets stretched. Going up and down the map away from the equator the scaling goes from the circumference of the planet at equator which is at 0 degrees lattitude to a circumference of 0 m at the poles which is at lattitude of 90 degrees. The circumference of the planet at lattitudes in between those two is based on the cosine of the angle. Therefore, the scale in m per pixel is going to be: equator circumference x cosine(lattitude) divided by world map pixel width, in units of distance per pixel. And for example, the scale of the map at 60 degrees lattitude is going to be half that of the equator because cosine(60) is a half.
Other projections are going to be different but there is no way to get around the problem of representing a spherical world on a flat map without some warping. Most of the time, for role playing you just don't care as the player characters are not going to be traveling far enough over land for it to be noticed.
Hope that helps.
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