Materials and the process of making the instrument are even more important than the size or design in a lot of cases.
For an instrument made of wood or bone, the type of wood or bone will have a very large impact on the sound. Hardwoods vs softwoods, leg bones vs ribs will have a big impact on the sound because of density even before you get to shapes. Something as simple as a drum will sound different depending on what is used and how taunt it is stretched over the opening for the striking surface has a huge impact even when the rest of the drum is used.
Then there are things that can be done to those materials that affect the sound. Tension, small differences in thicknesses, and placement of holes will have an effect. Then there is the act of playing, which can be compared to how many different ways the same acoustic guitar can be played.
So when people try to recreate instruments from the past that aren't still common today, they try to find documentation or stories that explain the other parts beyond the looks of the instrument. They will most likely be close, but most likely not exactly the same way that the instrument was commonly played, unless that knowledge has been passed down over time.