Other questions like this one indicate that Unity tries to enforce a metric system, meaning that one unit should roughly equate to one meter. However, in VR specifically absolute size really does matter, because you may want things to be a certain size relative to you.
TLDR: is 1 unit in game space a meter in real space? If not, does there exist some realistic approximation?
I confirmed what mattnewport said in the comments: 1 distance unit in Unity is one meter.