I don't know either Oathmark or Oldhammer but the rules Warband remind me of most are GWs Warmaster rules or HotT.
Each unit is a single base. The base is important, the figures on it not so much, providing everyone knows what the base represents. Heroes and Wizards are a unit/ base.
For a basic game armies are probably going to be single digits of units. That said, if you have the time and space feel free to let your megalomania run wild!
It's a top-down, fast play system which I think gives a good game. Were I not so invested in Warmaster I'd be playing Warband. YMMV.
Hope that's helpful.