My kitchen is 12' 9" on the narrow side, which is walled. We are *just* barely fitting an island in because we will have cabinets on each side of the walls.

To give you some dimensions to work with, standard cabinets are 24" deep, you normally have a 1.5" overhang for your countertop, meaning that the countertop is 25.5".

You need a BARE MINIMUM of 36" for an aisle. This would be for a one-cook only kitchen. Most suggest your minimum aisle to be 42". Many say 48".

So, if you will have a configuration of standard base cabinet, aisle, island, aisle, and base cabinet, it's not going to work.

The breakdown:
(base) 25.5" + (aisle) 36" + (island) 28" + (aisle) 36" + (base) 25.5" = 151". You have 144" to work with.

Now, maybe you don't need the 2nd set of base cabinets to be as deep and you could shrink them down to do a tall, shallow pantry that is about 12" deep. But, a standard configuration isn't going to work in your space.

I haven't seen your floorplan, but I honestly think you would be better served not planning on an island.

In my old house I had a moveble island, it was 26"X 48" The room was 12 X 11.5 This was 20 years ago. You could get the dishwasher door open and on the other side you could open and get in the side by side fridge. There was little usable counter space in this kitchen and I loved this island. Very handy for one person in the kitchen. couple years later we found you could slide it around -dont ask- so we turned it 90 degrees and viola different kitchen! But it was handy.
Is your kitchen 12 X 16 Cabinet to cabinet? Or drywall to drywall?

our cabinets will be laid out in an L, so the breakdown would be (base) 25.5 + (aisle) 36+ (island) 28" + (aisle) 36 = 125.5 which means I could widen the aisles to 45 or so. does that sound right?
its hard to visualize because we havent done the demo yet so the space doesnt actually exist, but maybe I will mark off the aisle in my current kitchen and see how that feels.
thanks for helping me with the dimensions, I will continue to play with this.

also, what is the smallest width island that works? I dont want it so small that it looks silly.

If an L, then it could work for you. I'm still pre-reno, so I haven't lived with our 36" aisles yet. To be honest, it does concern me a bit. But, I really need the island for counter space.

Are you planning on trying to put seating at your island or is it just a storage/prep area?

It's really a matter of opinion as to what the smallest width is that works. You really wouldn't want to go less than the standard 24" base cabinet. I think that the appropriateness of the width depends a lot on your kitchen width. For example, I think it would look really silly to have a 20' wide kitchen with standard cabinets and a 24" base island in the center.

Our island will be (probably) on a 26" base cabinet. We're still debating on if we should go back to standard 24" base and give the 2" back to the aisles or not. Guess we're going to need to decide for certain really soon! I anticipate our aisles looking a little tight, but I think our island will look "size appropriate" for the area.

Also, I think you need to keep length in mind as well - I think that keeping things near 1/3 dimensions works nicely - so an approximate 1:3 ratio or alternately a 2:3 ratio. So, if you have a 24" wide island, I would probably try to keep it to somewhere around 66-72" to give a eye-pleasing long rectangular shape or I would try to go to a more square rectangular shape so that you have a 24" x 36-42" (I think the second option would be too small looking in general, but wanted to give a real example of sizes to illustrate what I was talking about.)

I highly suggest drawing out your room and posting it on here to get some helpful feedback on ideas on laying out everything.

granolamom-
Don't know if you saw this or not but there was a thread very recently on islands and sizes. Lots of them shown were fairly large but not all, and there was talk of aisle widths as well.

Here is a link that might be useful: Size of islands