I went through a phase where I used two different sizes. I noticed that the kick and snare were much more prominent than the hihat/ride in pop/rock music, so I started using a bigger, heavier stick in the left hand. I did that for a few years, then got into a hard rock band where I ended up using the heavy sticks in both hands.
Like Joker suggested, I also mess around with rod/brush combinations, a favorite being a Vater Acoustick in my right, and a regular wire brush in my left. That's great for light pop/rock/country where you need to play quiet, but still want to "dig in" a little.
My motto in this area is
whatever works! 