I have been mulling over how to present systems of measurement in the default world of Heroes By Trade. Initially I thought I would default to the Imperial system just because it is familiar from DnD but my raw, unabashed hatred of it would not allow it. I then simply went with metric because it is better but I always felt strange about a fantasy world with such a sensible system that was derived from a relatively precise measurement of the Earth. For one, one ten millionth of the distance across the Earth's surface from the equator to the pole is not going to be the same distance on a fantasy world for any number of reasons - maybe the world is flat, or bigger, or made of waffles. My hatred of Imperial waged a long and terrible war against my sense that feet and miles had a more appropriate feel.
I could of course simply make up a totally new system but that felt like it would be clumsy. As Hobo pointed out, the GM is going to be describing things in terms of yards or meters anyway so if I make a system of measurement that doesn't use a familiar sized standard unit people will be confused and annoyed.
"So, the wall is fourteen dinglehoppers tall."
"Ummm, how high is that in feet?"
"Right, so the wall is eight feet tall...."
This really wouldn't get me anywhere. I had considered making the standard unit equal to the height of a human (or more likely a satyr, but those are actually very similar) because that would correspond reasonably to two meters / two yards but it would still end up with everybody doing conversion in their heads to a unit they were familiar with. It would be better, particularly since that would mean that the length of a square in combat would be equal to one unit, but it still isn't ideal.
Then I realized I was being entirely silly. This isn't a realistic model of the medieval world. There is a race of creatures called satyrs that were literally created by a being embodying Learning. Of course they would spend their time creating excellent, consistent, easily learned and remembered systems of measurement. Even if they didn't, the Being itself could have, nay, would have handed that knowledge down to them manually. The satyrs don't have the tools of modern science but they do have the desire and discipline to create and enforce a system of measurement that would supersede the whims of monarchs. After all, the children of those monarchs will be educated by a satyr!
However, I won't have the standard unit of measure be a fraction of a measurement of the planet. Nor will it be a distance travelled by light in a vacuum in a specific amount of time. It will be equal to the length of a bigass hunk of adamantium that is so thoroughly magical as to be nigh invulnerable. Hell, maybe someone will even try to dastardly plot to steal, destroy, or alter the Standard Bar and the characters can try to foil them to preserve the sanctity of the world's system of measurements. Of course the Standard Bar will be exactly one meter long.
Suck it Imperial!