The scale is 1 pixel (in full size image) for 1 tile in game. One tile in game is 1km, so to measure the distance you simply need to count the pixel distance between the two points.
Distance calculation is already present in the interactive map above, but in case it doesn't work, you can download the static map below and use an external software such as MSpaint, Gimp, or any online tool (e.g., RapidTables) to measure the distance.
World Map v678. (Left, Top) is (0,0). (Right, Bottom) is (8191, 4267).
Legends (need to see full size image and maximum zoom in to see the coast):
Blue : water
Ivory white : coast
Dark brown : mountain
Black : road
Dark green : forest
Brown : radioactive forest
Light green : swamp
Red : radioactive swamp
Light brown : wasteland
Grey : radioactive wasteland
World Map v678 with cities (WARNING: Possible spoilers ahead!)
The circles and points indicate cities and locations. Each city is to scale with the size in game, with possibly slight error. By default their color is white, but if there is more information on them, they will be color-coded.
Red area around Ufa is the bee zone, area where bees spawn. This is not an exact match to the ones in game, but close enough.
Light blue cities are cities close to a safe haven (while safe haven itself is blue)
Light green cities are cities close to a survivor base (while survivor base itself is green)
Pink cities are cities close to a bandit fort (while bandit fort itself is red)
Orange points are possible places for Looter's warehouse
Cyan points are locations related to Searching for Truth quest,
Purple points are locations related to Way Back Home quest
Light blue points are locations related to survivor base quests
Yellow points are other interesting locations.
World Map v678 with cities and zones
The zone number defines the various properties of the environment in that area, such as the radiation per hour, the possible loot, etc.
In older versions, these zones also define the kind of encounters the main character face. As of v674, it is now mostly based on character's level instead.