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One general rule of thumb I use for checking my inertia tensors is: If total mass of the rigid body is m, and the dimension of the corresponding body is d, then check to see if ixx, iyy, izz are near m*(d/2)^2.

This is by no means correct, but a sanity check to make sure the moment of inertia are the right order of magnitudes, so the model behaves somewhat physically realistically.

I've seen too many simulation where mass is 1(kg), and ixx,iyy,izz are also 1(kg*m^2), while the robot is only 10cm in diameter. This setup usually result in a robot that does not respond to rotational commands easily (given that izz of 1 means there is a 1kg mass at the end of a 1 meter pole).