Since you have the "test sink" now, try doing your prep on just one side and see if it feels right for you. And also test out other tasks that might end up in your prep sink, like filling a pasta pot because the other sink in in use, etc.

I suspect bigger is better if the space isn't being taken from anything else important.

I should mention, I have purchased the Karbon Kohler Faucet, which is pretty large, in case that makes a difference!

The faucet does make a difference--you can fill pots on the counter NEXT to the prep sink! :)

Oh! They have a 15"x17", maybe just right!

I would go with the 15x17. The 18x19 would be good too, but the 13x17 is a little narrow for being able to set pots or colanders in. I also like being able to set a dishpan inside mine from time to time.

IMHO you want a prep sink large enough for a 25 pound turkey. Otherwise you have raw poultry in your clean zone.

Second julikcmo's expert advice -- you really want to be able to set stuff in there and let it thaw, as necessary. Or, be able to wash a watermelon, turkey, etc.

First, thank you all so much for weighing in on my decision!

A different question, I just found a 14"x16" (basin size) in the enameled cast iron that would be identical to my main sink (kohler Whithaven in cane sugar). I had planned to do SS for the prep, but am now second guessing and considering matching the sinks...

So, is it too matchy matchy to have the sinks the same? Or would it look better with them the same?

Thank you!

Not too matchy matchy. But okay to not match. How helpful is that? Not very! But I think it would be nice to match.

While it's nice to have room for a watermelon or turkey, I wouldn't give up that much room for a prep sink unless I had unlimited counter space. My main criterion was that a large colander and largest pot would fit. As for raw poultry in the cleanup area, that wasn't (and still isn't) a concern of mine - and I'm still alive :-)