There is no substitute for cubic inch or in this case surface area

How does sound stage and woofer size go together? I thought that is more a characteristic of mids and highs. 

I think of multiple small woofers only in the sense of how HIGH they can go. Given the same volume (moved air), the key to me is that I can run an 8 woofer much higher than a 12", potentially allowing a much simpler 2 way crossover with all the potential negative impacts from any part in the signal path. 

What I don't know is if (given the same air move volume) if four 8 inchers can reach as far down as a 12" (SPL at low frequencies, and resonance frequency). 

Directionality increases with frequency.  Larger midranges will be more directional so placement matters more.  My experience has been that I prefer speakers with a smaller midrange.  They tend to have more even in-room responses.