There has been some confusion on whether we're going to use feet, centimetres or unreal units. It doesn't really matter which unit system you're using so long as the final asset will be correct in unreal units. Here's an article that sheds some light on conversion ratios: http://wiki.beyondunreal.com/Legacy:Unreal_Unit
If everyone would like me to, I'll make a plane with a texture on it depicting average human height along with other various other objects of descending scale. Sort of like the texture that Mike showed to us that Valve Corporation had used.
As an afterthought, perhaps it would be best if we all just used unreal units (2.54cm) for the sake of simplicity. We can even convert our existing asset measurements into UUS to make understanding scale less of a hassle. What does everyone else think?