I could always resort to the units of measurement still popular in the States (and used in Dungeons & Dragons and its ilk), many of which are appropriate to a medieval setting (e.g. ounces, pounds, feet, yards, miles, etc.), but this renders the rules a little less accessible to gamers more accustomed to the metric system. What to do?
The solution is to use a system that is equally unfamiliar to everyone. Allow me to reintroduce the cubit. Based on the average length from the elbow to the tip of the middle finger, the cubit has varied over time and from nation to nation (the ancient Egyptian royal cubit was between 20.6 and 20.8 inches, the Aztec cubit was 20.7 inches, the ancient Chinese cubit was 20.9 inches, the ancient Greek cubit was 18.2 inches, etc.), but it has commonly been rendered as 18 inches (or 46 cm), especially in the Old Testament, and this is the cubit we shall be using.
The 18 inch cubit is perfect for fantasy gaming use. It does not evoke mood-breaking modernity, yet it is easy for gamers to translate into units they understand. For the U.S. gamer, a cubit equals one and a half feet or half a yard. For most other gamers in the world, a cubit equals approximately 0.5 meters. If the rules were to use cubits, it would make them truly universal. International gamers would not need to be annoyed by movement rates of 120' and U.S. gamers would no longer need to argue about whether 12" was 120 feet or 120 yards and why it was being expressed in inches. (Yes, I know the answer to that, but I'm trying to make a point rhetorically.) For the sake of simplicity and sanity, let it be cubits.
1 cubit = 16 inches = 0.5 yards = approximately 0.5 meters
Do you need a unit of measure smaller than a cubit? The Old Testament cubit also provides the answer, being divided into 6 palms of 4 fingers each for a total of 24 digits.
1 cubit = 6 palms * 4 fingers = 24 digits
So, a palm equals 3 inches and a finger equals 0.75 inches. At any rate, when in doubt, one can always just use one's forearm, palm, or fingers to estimate!