Does anyone know what the size of the loading coil that MFJ uses in their 80
& 40 meter vertical?

Dentron, The size of the loading coil is not so important as the inductance
of the coil.
What I would advise is put the best ground radial system you can. Beg,
Borrow or Steal an antenna analyzer and determine the impedance of the vertical
at the frequencies you wish to work.
Then use the Smith Chart, to determine what L network configuration will
provide a match.







73 Gary N4AST