I'd advise against it. We have done a lot of playing with home brew rules that are evolutions of WM, in one of these we tried variable sized units, and it proved very hard to balance.
Similarly in Kings of War which has 3 key sizes of units, in the second edition they extensively revised the costs and purchase limits for balance.
I'm not sure you will gain a lot game play wise, but it will certainly cause a lot of complications in balance.