I actually would split the 12" between the existing drawer base and the sink base. I think a prep sink should be at least 13" up to maybe 18" to be truly useful. Think about wrangling your largest stock pot into an 11" sink. Or I like to take greens like mustard or collards and drop them in a half full sink to clean the grit out of them. A full bunch of greens takes up a lot of space. YMMV.

I think splitting the 12" between the drawers and the sink base is a good idea. You could make the trash pullout smaller, but a 12" sink base doesn't give you room for what I would want to be a useful prep sink. You need at least a 12" sink width (and that would be with a small radius sink), and 13-15" is better if you plan to put a colander or large pot in it. I wouldn't make the sink base less than 18".

If you are thinking of making the doors inset, you'll need to go with fewer and larger cabinets. Those 18" drawers are going to get a lot smaller. Make sure they are going to be useful in the spot planned. Have you thought about what you need to store in that space to be useful and functional?

I agree with laxsupermom ... make your prep sink bigger, you won't regret it!

LOL. That's my first thought to: That a sink small enough to get into a 12" space is usually a 10ish sink... too small!

Definitely a bigger prep sink. I'm probably really extreme on prep sink size but my sink base is 24" which also accommodates a big garbage disposer and a Never MT soap dispenser. I forget now the exact sink width but I think it is 17". I also soak big batches of greens---collard, mustard, etc.---and dump big pots of pasta water into it. My SIL had a small prep sink in a 12" base in her old kitchen and it was the one thing she regretted b/c it wasn't big enough for anything except hand washing or getting a glass of water. HTH.

Maybe a 12" spice pullout. I have one in my bar on the end and my cooktop is to the right of it and my double ovens to left on the other wall.

I would go to an 18" cab for the prep sink and increase the size of teh 18" drawer base to 24".... pretty much what others said.

Ditto...

Prep sink interior dimensions should be no smaller than 12" square, 15" or bigger is better...an 18" sink base is best so you can fit a bigger sink...

The opening of an 18" framed cabinet, inset or overlay, will be 15". That means your drawers will be less than 15" wide. On top of that, you're putting 4 drawers in, which will mean that most likely none of them will be that deep...unless it's like ours with the top 3 the same size and the bottom 4th drawer deep...not as deep as a deep drawer in a 3-drawer base, but still deep enough to be a bread drawer.

Unless you have specific items planned for that 18" drawer bank that you know will not need any more space, I would make it 24". If you absolutely do know you will not need any more space, then put in a 6" filler pullout!

One other comment...I would rearrange those cabinets so the sink isn't smack dab in the middle of the island. It breaks up the wonderful expanse of workspace an island offers. I would do something like this:
18" trash pullout cab
18" prep sink base
24" drawer bank
24" microwave cabOr this:18" prep sink base
18" trash pullout cab
24" drawer bank
24" microwave cabOr this:18" prep sink base
24" drawer bank
18" trash pullout cab
24" microwave cab

Think about how you work. Do you want to stand at the island and have the trash pull out open as you're working? Where is that most comfortable? Left-handedness/right handedness comes into play here. Same with the sink base, as you stand there, where does it feel most comfortable - to the left or to the right? I'm left handed...to me, what's most comfortable is that I'd want the trash to the far left, so I can toss things in there as I work, have the entire center of the island as work space, and the sink base all the way to the right. The reverse works just as well also (trash to the far right, sink to the far left. I wouldn't center the sink base, it would kill your work zone. Bottom line, I'd make sure that I had some work space between my trash and my sink so I could stand and work between them and use both features.

Thanks everyone!! I'm going to rework and see what I can do about sink size; I was trying to preserve the sink being in the dead center of the island just because I liked it there aesthetically, but I'll get out the graph paper and see. So if I want to try to keep the sink in the middle and larger (say 18" sink base), I would have

18 trash
15 drawers
18 sink
9" (either vertical storage or a 6" and 3" pullout)
24 microwave

losing some valuable drawer space there, though. Should I nix the hopes of inset in the island (still holding out for the rest of kitchen?) in favor of space? In that case, I can reduce size of trash to 15" (I think) and get my 18" drawers back.

I think that, functionally, you'd probably like the sink better off to one side. You'll appreciate having more room to spread out much more than the aesthetics of a centered sink. I know I do! If you move it off to one side, though, it would look better way off center to keep it from looking like a mistake.

What should be where really depends on the rest of your kitchen. For example, the trash pullout should be easily accessible for both prepping and clean-up. The microwave should be close enough to the fridge that you can pull something out of the freezer and pop it right in. What's around your island?

I'm a very symmetrical person so I understand why you may be leaning towards centering your sink. However, there is a time where function trumps form. Your work zone will be chopped up if a sink lands in the middle. Would you rather have 66" (about 5 1/2 feet)of clear, unobstructed space to work with or two 33" (not even 3 feet) spaces?

All right, I'll say it...for resale purposes, I can see this being a negative if the sink was centered. My current kitchen is currently plagued by limited, small, scattered work spaces. I hate it. I'm always running out of room. One of the reasons we're remodeling - to lengthen and streamline any available space.

Thanks everyone for your thoughts on this matter; I really really appreciate it. Here's the deal: plumbing is a done deal, so the sink is going to stay in the middle. I do have a 40" wide, second work/prep zone on the perimeter in front of a window w/ no upper cabs so that helps with the addl. prep area. I COULD add 6" to one side of the sink, making the island 7.5', but any more than that will cut into a high trafic thoroughfare.