Scaling forces, impulse, mass and drag coefficients would give you the exact same simulation in the same dimensional scale. Scaling the size of the geometries can also have it's advantages.
As for MOI (Moment Of Inertia), it depends on mass and radius (I took circle as an exsample). If you scale down the mass, it will automatically change the MOI, changing radius will have the same effect.
It basically comes down to this:
Scaling the forces, impulses and the like will result in lower numbers for the solvers in the engine. Lower numbers means lower possibility of catching a NaN (Not a Number).
Scaling the size of units (and force, impulse and the like) will make it possible for you to zoom in your game. We have the ConvertUnits class in the engine to help you quickly convert between simulation units and display units. We use iterative solvers
that come to a conclusion over several steps. They approximate the real value instead of determining it right away. We need to do that because it gives better performance. Iterative solvers (in our case) collect some kind of error that we would like to be
as small as possible. We also collect values from the last step to use in the current for faster solving (called warm starting). All of this can benefit from scaling in some way or another.
It will become more apparent in Farseer Physics Engine 3.0 were we use the MKS (Meter Kilogram Second) system. The algorithms are tuned to use meters and using pixels directly can give an unstable simulation. Here it will be needed to scale everything.