As far as I know, there is no problem going smaller (unless for some reason you can't go more than X% smaller because then it would be too small to rest on any bracing or support bars in the cabinet?)

My understanding (which may be wrong) is that the sink is clipped to the underside of the granite (or other stone) countertop. So it can't go bigger than the inside dimension of the cabinet (minus a bit for the clips), but it can certainly go smaller.

It likely depends on the kind of sink and the countertop material you are planning to use. If it's a drop-in type, it doesn't seem like you're limited by size at all - it just needs the appropriately sized hole in the counter. If you're going to undermount, you have to think about the structural bracing issues as noted above. Either way, it doesn't seem like an issue any competent contractor/carpenter couldn't handle. :)

Thanks all -- this is exactly what I wanted to know. I didn't explain it well, but I suppose I was wondering if the stone braced the sink or the cabinet... now I know! :-)

Have you already ordered your cabinets? If not, maybe you should consider having a smaller sink cabinet and a larger adjacent cabinet. Sink cabinets do not typically have great storage space. I wouldn't go smaller than 30", though.

On the other hand, if you are planning on keeping trash and recycling under the sink, you might appreciate the larger cab.