I have no way of being sure so I'm just going to base this off a massive amount of assumptions and averages.
Assuming all the playable races are the same weight, and since they are the same height,
1 "Height" unit is approximation 71 inches.
Assuming all the people in skyrim are pretty healthy and fit, their body mass index is probably about 21, with a height average of 5'11, their weight must be roughly 150lbs.
So in my completely non-official, non-canon opinion, 1 unit of "mass" is equal to 150lbs.
I dunno. It's what I always go off of.