All of these measurements depend on the converter you use as well as the scale setting used when exporting. There's the Destiny's old obj2blb, the newer 3dtoblb, Badspot's BLBConverter, and General's private one.
The ratios of a 1x1x1 brick are 1 unit wide, 1 unit deep, and 1.2 units tall. A 1x1x1 plate would be 1 unit wide, 1 unit deep, and 0.4 units tall.