On wiki, you can find these approximations:
UU=Unreal unit
256 UU = 487.68 cm = 16 feet.
1 meter = 52.5 UU
1 foot = 16 UU
1 cm = 0.525 UU
1 UU = 0.75 inches
In max, if you make a 6.52x6.52x6.52 meters of a box, it will match exactly 256x256x256 of a basic unreal room.
However, it's not suggested to go crazy about precise dimensions. For instance, according to upper list, if you want to make a 2 meter room don't subtract a room of 105x105x105 as BSP needs powers of two. Even I'd not subtract a room of 106x106x106. I'd do 128x128x128, since it's much better for bsp. So in general, subtract room sizes by powers of 2 and then "add" static meshes to lower the dimensions on ceiling or floors. You will have less headache.
check out this link for more info
http://wiki.beyondunreal.com/wiki/General_Scale_And_Dimensions