I've learned this lesson fast: it depends on how big of project are you planning? An appropriate block needs to fit both scale and design wise. You don't want a block to get lost or overwhelm a quilt and true the two tones are of major consideration either way, but first things first. You can always scale a pattern to fit, but some are naturally inclined to a particular size. I mean you don’t want to make a triple Irish and miss the pattern making a tiny quilt. You might also want something graphically strong since you are working with only two colors.
I'm fairly new at this so I might be totally off on the observation.