Nantucket sinks makes a 16x11. That's what I purchased for my 48 x 18 vanity. Check with your fabricator if using stone on minimum size in front of and behind sink. Also vanity cabinet specs if available.

You could consider using one of those basin sinks, which rest on top of the counter. that would maximize your sink size. Another possibility is putting the faucet on the backsplash area, which is very common and popular now, either with a basin type, or standard style. I'm dealing with a similar issue in my planned renovation because I want a narrow counter to open up the space in the bathroom.

To be completely honest, I'm not particularly a fan of the backsplash faucet because I just know that sooner or later my faucet would need some work that would involve breaking the backsplash tiles. But I'm the queen in the land of Murphy's Law. Other people are very happy with that style.

A Corian top with integral sink would be ideal for your situation.

What model Kohler sink is that?

We are thinking about Corian as well. I might look at the illuminated line to act as a night light for the kids.

In a similar situation but going for a single sink setup. I'll double up the vote for the Nantucket.

http://ths.gardenweb.com/discussions/3223273/minimum-countertop-depth-for-a-given-sink-faucet-combo

As you search for a sink, be sure you don't neglect the depth. In our old house we had very simple sinks, but I loved them for their function. Our new house has lovely, shell-shaped sinks ... but they're too shallow, and they splash water back up at us.