The Sun has a large , of order 10.  Dissipative affects are generally small, and there is no difficulty in maintaining a magnetic field against diffusion.
For the Earth,  is estimated to be of order 10
Dissipation is more significant, but a magnetic field is supported by motion in the liquid iron outer core.  There are other bodies in the solar system that have working dynamos, e.g. Jupiter, Saturn, and Mercury, and others that do not, e.g. Mars, Venus and the Moon.
The human length scale is very small so that typically . The generation of magnetic field by the motion of a conducting fluid has been achieved in only a handful of large experiments using mercury or liquid sodium.