There's two differents things, here:
Units: There no optimum unit in Raydium, just keep "correct" ratios (for size, mass, speed, ...). For sizes, test6 and other demos are using something like "a car = 1 unit", for example.
As a side note, i'd like to say that using "realistic units" for a game is not a good idea (it's very hard to tweak), and the other important point is that computer's FPU does a better job when numbers are "around 1" (float accuracy is way better).
Blender and scaling: triEXP* export scripts exports meshes, not "meta-data". So when you work in "object mode" with Blender (and not in "vertice mode"), remember to "apply size/rotation" (CTRL+A) before exporting. The best way is to use only "vertice mode", I think.
PS: your "linux games" project is interesting