My island, which I use for prepping, is 36" x 70" (3'x6'). My prep sink, interior bowl size, is 14"x16"". I wish I had gone with the 18"x18" (interior bowl) that my KD suggested. I can fit a colander in it but it's on the small side for draining a large pot of hot water (pasta, etc).

can you mock it up with cardboard to get a feel for it? You could set $ store bins of various sizes on top to try out. I taped my island out on the floor which was helpful, but it seemed really puny to me. I was telling my friend how small it felt and she said, "yah, that's cause it's make out of tape". We stacked up some bits of lumber to size and that gave me a much clearer idea of what the size would feel like.

Have you considered round? We have a 16" round prep sink on an island of similar size. It's on one of the long ends. I wanted to bring in some "softer" elements into my mostly angular kitchen, which is why I chose round. It's plenty big for all of my prep needs. I think you do want bigger than 15" square. I wouldn't want my prep sink to be any smaller than it is. I don't think either of the last two you mentioned would be too big, but I probably wouldn't go any bigger.

There's really no reason that the sinks have to match in shape. The sinks have different functions and can be shaped appropriately for their individual jobs. In addition to size of sink, spend some time thinking about placement. Many of us love things to look centered (raising hand, "Me, me!), but for function, it makes more sense for the sink to be off center to give you the best prep space to one side or the other of the sink. I hope you'll share which sink you land on. : )

I also have a 8' island, but it has my main sink in it. I can't tell you what size of sink to buy, but I can tell you that you definitely want it off center. Think about what you want/need to be doing at the prep sink and also where your main sink is in relation to your stove and your prep sink (it may be just as easy to dump the pasta water in the main sink?). I wouldn't want my prep sink any larger than necessary because having a cramped prep space is not fun.

Our island is 4x10, so I can say that as long as you don't center the prep sink (keep one sink closer to the range, and one closer to the refrigerator), any size will do - even a full-size sink if you were so inclined, because you have plenty of room. That said, we had a 14"x16" in our previous home, and it was a very good size for us. I'd highly recommend putting a garbage disposal in the prep sink.

thanks all! some really valuable feedback! Our main kitchen sink is going to be an undercount rectangle about 30" long. that sink as well as the prep sink will both have disposals and the prep sink, which is closest to the range is definitely off center to allow for maximum prep space. i never really considered a round sink because I felt the edges had to flow - haven't settled on main sink yet but its between a franke and a Blanco right now bc they both offer very sharp edges with a non zero radius edge vs a more rounded look, which i don't particularly like. I will certainly post when i have made my decisions!