And what, exactly, is 'width', as modelled in HoI4?
A standard division with three regiments each with three battalions, would NOT in most cases have all nine combat battalions on the front line. The normal would be to have a sizeable reserve, either one of the regiments or one battalion from each regiment, which would leave six battalions on the front line. Even less if you consider having reserves at all echelons. Artillery would NOT contribute to this width, since it would be grouped to the rear. Same with A/A, although AT might add some width. Armour would as well, although most likely less than infantry.
But is this 'width' really a limiting factor on the geographical scales that we have in the game? I'd say no.
I think what we're really trying to model is logistical congestion in a province, which is a real limiting factor in real life. The infrastructure can only handle this many vehicles before everything grinds to a halt, and although combat units can maneuvre outside of roads, the logistical tail of a division cannot in most cases. And this tail is huge. Artillery units will have a larger footprints than infantry due to the large amounts of ammunition they need. The same with armour units, due to both ammunition and fuel.
Implementing a system limiting the number of units in a province, as opposed to each front in the province, would be more realistic. We already have the supply system, of course, and the easiest solution might be to enhance this by increasing penalties so that units are under-supplied earlier.