I will definitely concede that commanders looking alike is very annoying, as I've made more than a few recruitment errors myself. In the end I just learnt to double-check when recruiting commanders with certain nations, but still the odd mistake creeps in. And agree that MA Caelum is one of the worst. Plenty of scope for recruiting the wrong commander there.
Well, now that we've all thrown our opinions around a bit, I'll say that the unit size really isn't a big deal and that, as Burnsaber suggested, modding the units would be a simple fix to distinguish them better.