12" x 12" is probably the minimum I would go for...but if you or your DH have big hands, I'd recommend bigger. Mine is 15-3/4" x 15-3/4" and I wouldn't want it any smaller...but I have long fingers & relatively wide hands (for a woman) so I may be biased in that respect. [However, my hands are b/w a medium & large Men's gloves so they may be "average" for a man!]

Find boxes the sizes of the two sinks & pretend to work in them...it should give you a good idea of which sink would be better.

I have a Blanco about the same size as Buehl's -- I call it 15x15, but I'm not sure what Blanco calls it. I think 12-13" is minimal to be useful. I prefer just a little larger to allow me to set a large pot or colander in the bottom. Mine does and would fit inside and 18" cabinet although what we did was use a 30" base and have it to one side rather than paying for two smaller cabinets. If the sink is not zero radius, keep in mind that the rounded bottoms will prevent larger things from sitting flat in the bottom of the sink. Don't be shy -- take a colander &/or pot with you and go look at sinks. You may have to test the smaller bowl of a double bowl to find one the same size on display.

Mine is 12" round. I can't wash a watermelon, but that's the only thing that doesn't fit. The colander thing is very important. I have two colanders, one small with a long handle for berries & such, the other large. Both sit well on my sink so that they can drain things that have been washed. They really increase the functionality of the sink.

Try to think of all the specific things you'll use it for and determine the size you'll need based on how you cook. What makes sense for me may not make the most sense for you.

We have the Ticor 13"x16" sink you mentioned (S805), and we love it. It fit in our 18" cabinet with plenty of room to spare. It has turned out to be the perfect size for every use we've come up with so far. It fits our largest colander, and it fits whole celery stalks. I can soak lasagna noodles in it, and I can put a large bag of ice in it during a party. I also LOVE that it comes with a sink grid, because I use that all the time when I'm prepping.

I can't imagine that losing an extra inch or two of counter space would make that big of a difference, but losing the inch or two inside the sink just might.

I also have the Ticor s805. It is much improved in functionality over the 12x12 sink we had before. It is a great size, and you can get even gets big pots in there. I have an arched faucet that makes filling easy (oh, get a spray feature at the prep sink and a disposal too!) The grid allows you to use the whole bottom of the sink.

I have an 18" I can't imagine anything smaller being useful in my world of two cooks, and honkin big pots.

We have a medium sized Blanco Precision 13"wide x 18" depth (it was tight in a standard 24" deep cabinet but we made it work) x 10" deep. The cabinet is 18" wide. My wish was to minimize the amount of counter space interrupted yet have a perfectly functional sink for all tasks and I feel as though we achieved these goals. It was our only sink for about a month before we got the main sink in and I found I could do almost anything with it. We've had it for 4 years and I've never wished for it to be smaller or larger. Gaining a prep sink was one of the things I most looked foward to with our new kitchen since the one sink tended to be a roadblock in our old kitchen.

I wanted to plan our kitchen for multi cooks and multi clean up people (wishful thinking). We hand wash a lot of our frying pans and big pots and old pots with plastic handles. I put our dw at the prep sink so that one person could be processing the dirty dishes for the dw while the other person was at the main sink hand washing. And in an ideal world there would be a 3rd person attending to left overs.

I have the Blanco round 15" diameter stainless steel undermounted in an 18" cabinet. (Blanco, of course, had recommended a bigger cabinet). The edges of the cabinet had to be eased down just a hair for the thickness of the sink edge. It's a very functional size and the round takes up less visual space than a rectangle. I think 12" would be uncomfortable. I too have a smaller kitchen and the second sink makes it more functional with much better flow.

This is incredibly useful - thank you so much everyone!!!

Buehl and lastcatx - oohhhhh, yes, setting up mock-up box and taking the colander to a store are perfect ideas. (amazing how obvious something seems _after_ someone kindly points it out to you :)

debsan- your watermelon made me laugh!

holligator and kidshop - it's so helpful to know that this sink works for you the way we want it to work for us. (I love how people know that serial numbers of these products - me, I can rattle off a whole bunch of faucet numbers now)

pirula - Good point - We do have some seriously honkin pots! I hear you on the multi-clean-up and multi-cook wishful thinking. I'm beginning to think, though, that since DH is a fabulous cook, our 2-person kitchen design may be less about me getting to be in the kitchen to help prep/cook, than me getting to be in the kitchen to help keep it clean....

zelmar - good to hear the balance works for you - it's been a real roadblock for us too.

judeny - it's relief to hear that someone else finessed the edge of the cabinet - I've been saying "oh, we'll just shave off/cut out the sides of the cabinet," and I've started to worry that I'm just delusional.

again - thank you all!!

Great tips! I'm planning my island prep sink and was thinking of those small narrow ones but now I'm re-thinking.