The Python API misses a "dimensions" property on KX_GameObject. This patch addresses this issue. I don't know if it's the best implementation, but it works.
There are two new functions now:
KX_GameObject:NodeGetLocalDimensions used to create the new Python accessible variables:
worldDimensions (the dimensions of the object on x, y, z relative to the world axis)
localDimensions (the dimensions of the object on x, y, z relative to the local axis)
Both new attributes are of type MT_Vector3.
Has been discussed at https://developer.blender.org/T42017