Given that ther RAF's traditional structure was based upon the Cavalry (courtesy of the Royal Flying Corps), convention dictates that its nomenclature should be the same as that of our green-suited cousins. In that case, a force or army group should consist of 45,000 or more men, and a corps should consist of 20,000 to 45,000 men. A little tenuous, I suppose, but that's one viewpoint.
"Royal Air Corps", anyone? Apparently we get mistaken for the RAC anyway, so why not change it!?