I’m not familiar with units in Blender, but in three.js the convention is that 1 unit = 1 metre.
So if you create a 1-metre cube in Blender, and export at a scale of 1, then you should get a 1x1x1 cube in three.js… assuming that the Blender exporter and the FBXLoader are both correct.
I’m planning to examine whether the FBXLoader is loading objects at the correct scale at some point. At the moment I have a suspicion that it is not, and I have no idea about the Blender exporter.
If you want to investigate and provide me with some data that would be great 