Back in Brawl, people often used stage builder blocks (or SBBs) to measure distances, but now that those are no longer in the game, I haven't seen a standard form of measurement yet. Some people still use SBBs to measure distances, since you can just fire up Brawl's stage builder if you ever need to get a point of reference again, but then you can only make rough measurements instead of exact distances.
I've seen a unit called Falcons be used to measure distance across stages, which is the distance Captain Falcon travels after using his U-tilt. This method makes it easy to calculate just how far a distance is down to a precise unit, but it only works on flat ground, and hasn't really seen very wide use or universal recognition, so many people may not know what you mean by "falcons".
The most common way people measure distances is by comparing the percentage needed for a certain move to send a certain character that distance, but there is no standard for which move, character, or what settings to use for the testing.
I am not aware of any standard units or methods of measuring distances in this game, but as long as you explain your scale it shouldn't be a problem. However I agree that there really should be a standard, universally used unit of measurement in smash 4. I don't know what it could be but it would be very useful for the technical side of the game.