So this is for a little question I just answered asking me to come up with a new method of measuring horses instead of hands. And since all flutes these days produce the same sound, they must be of equal length (There are probably different kinds of flutes that are different lengths on purpose, I'm not sure). But then I started to wonder if this has always been the case or if it's just now a product of our standardized measuring systems (Not many things are measured in hands anymore). I'm assuming they hand crafted their flutes, and so there must have been some significant differences in the sounds and sizes of each flute, if not between each flute maker? Or is this not the case and all flutes were the same size? If so how would they have done it.
I did a fairly extensive post on the history of the flute here, if you're interested: https://www.reddit.com/r/AskHistorians/comments/3ec872/i_have_read_j_j_quantz_revolutionized_the_flute/
But that mostly deals with post-Renaissance flutes.
In the middle ages, there was little to nothing in terms of standardization for musical instruments. The main reason instruments are standard lengths now is because of tuning - we want groups of instruments to all be able to play in tune together on the same pitch at the same time, so we've determined that concert A=440 Hz, and all instruments are based on that assumption.
However, before the baroque era, there was little to nothing in terms of attempts to standardize instruments. Partially because musical instruments were almost exclusively used in a secular context, and partially because there weren't large ensembles of instruments playing together that required a standard tuning.
A medieval flute was essentially a wooden tube, closed at one end, with holes drilled into it. This is assuming you're talking about the transverse flute. That is, a flute played perpendicular to the body. A recorder is also considered a flute, but generally not referred to as such very frequently now.
So, makers would generally pitch their own instruments the same (or as close as possible with pre-industrial fabrication techniques), so flutes from one maker would be the same length as each other. And a maker would typically make smaller and longer length flutes - similar to a piccolo or alto flute now, pitched up or down, but again, consistent with each other.
However, when you compare flutes between makers, there are subtle differences. some would be drilled a little wider, or shallower, are a little longer or shorter, or vary the placement of the holes. If there's a joint on the flute, those would vary in style and materials, whether just carved wood, or waxed hemp, or ivory.
Now, all that being said, the differences wouldn't be vast, unless you're comparing across continents. Flutes in the same region would be made in a similar manner, and would be reasonably able to play with each other in tune, but get a flute from Northern France, and try to play with one from Southern Italy, you're going to see bigger differences (assuming there were any eventuality where someone would ever travel this far with an instrument and want to jam out, which would be pretty rare).
