In every game the quantity of distance is labelled in units.
Sometimes these units are translated to more regular length quantities like (most used) yards, miles or meters (lightyears tend to be used a lot in space games).
Throughout the entire game, it should be using the same values for the same lengths unless pointed out differently. This to avoid confused (and therefor) fewer users.
In WC3, one unit is about 2 to 3 centimeters (according to basic unit lengths).
In velocities in games, they ussually talk about x units per second.
Whereas a speed of 900 will cause the projectile to move 900 units per second.
Buildings are placed in a grid of 64x64 units.
Pathing maps can be on 32x32 tiles (where 1x1 pixel is 32x32 units, but can be used with a half tile offset if used in an odd number).
Ranges, movement speeds, flying height, etc all follow the same distance formulas.