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I know that SolidWorks can calculate this value for you, and I'm assuming that any other CAD program can as well.

As a general rule of thumb, I try to follow a number of guidelines when creating urdf models. These are mostly based on experience, so they should only be taken for what they are.

1. Don't make objects too large
2. Don't make objects too small
3. Avoid modifying the gravity
4. Keep masses for dynamic objects as close as possible. (i.e. the simulator will not like it if you place a 1000kg weight on four 1kg wheels.
5. When in doubt, use the identity mass moment of inertia <1 0 0 1 0 1>

You might also want to try the following to make your links more "solid":

<gazebo reference="back_left_wheel">
  <kp>1000000.0</kp>
  <kd>1.0</kd>
</gazebo>