You've already got a nice collection of links in the comments and in answers to get you started.
Measurements during this time were not consistent from place to place, however, traders did do conversions, letting buyers know how a general measurement which was used more widely, stacked up against the local measure.
There are many complaints based on measurements in the annals of law during this time because of the inconsistency. In fact, a person selling cloth might even have their own measuring stick (which might just be a single, flat stick) that they might use as a measure, telling customers that each of those is a unit, with the varying cost attached.
About 1200 or so because of the rize of Medieval fairs, more interest in consistent measurement, mandated by local rulers became more prevalent. So, at the Troyens Hot Fair in Champagne for example, they mandated that all merchants use a standard of measure that they imposed. The Champagne ell (2 feet 6 inches) was the standard and was checked against the iron bar wielded by the Keeper of the Fair. The fairs in that region were popular enough, with enough goods coming through, that this method of measurement spread to other regions in France and Belgium.
But this doesn't mean that an ell in France was the same as an ell in Germany or England. There were literally hundreds of thousands of measurement units, and in less regulated places this could even vary from shop to shop.
England, perhaps because of its isolation, kept the Medieval system the longest. America's current measurement system of pounds and feet and gallons--is actually closer to that old Medieval system than the Metric system is, at least in terms of what the measurements were.
While there were provincial measurement systems in England, the places that traded the most were the ones that set the standard. So, London would actually send out measures of bronze or brass to the provincial towns to basically let them know what the standard was. They would then "copy" those and send them out to other villages which wanted them. By copy, I mean that they would counterweight something else, such as sand in a container of equal measure. Unfortunately, these copies and copies of copies were not always accurate (because means of measurement were not always that good). So a pound in one place might not be quite the same in another. (Jokes could come from this--like a town known for their pound being short, if the word spreads, can become a saying-- people might know that a Jameston pound will always come up shorter than a London pound).
Standards were reissued in the hopes of keeping things consistent, but until Elizabeth I in 1588, a uniform system in England was not well imposed. (Okay, there was an effort in 1496 as well).
If you are starting from scratch, I would use the American system as the guide, because it's the closest thing we have in modern times to what the systems were like then, rather than the metric system.