We did this - used a backsplash only behind the sink between the counter and the window. We removed the trim below the sill so the piece would fit snugly against the sill. For length, we went 1 1/2 inches past the sill on each side, which created a nice graduated effect with the sill edge and the vertical side trim above the sill. We have a single window with a radius top. Sorry I'm out of town so I can't take a picture today. We're very glad we did this as there's a lot of splashing back there.

Thanks, I would love to see photos when you have a chance to post.

Localeater, here are a few pics. Brace yourself if you have a strong aversion to oak cabs. :)

here is the backsplash with window:
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and here it is a bit closer:
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We got this piece in a 2cm thickness; the countertops are 3cm.

I would match the width of the windows, as shown. If the backsplash was narrower, the window would look unstable,

Thanks all. Zen I really appreciate the photos. If you don't mind, how many inches is it from your countertop to the bottom of your window sill? I think mine is a taller opening at 8.5 inches but mine is also much longer. I just want to make sure it looks balanced, which yours does and does so well. I also found this picture on line. Of course I am getting an undermount sink not apron front, but it does show the longer window and I think taller space between counter and sill.
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Yes, that picture definitely shows a taller backsplash. We got the backsplash piece 5 3/8 inches tall - the windowsill is about 1/8 inch higher than that at 5 1/2 inches above the countertop - in order to leave room for sealant and caulking.

I agree with lazygardens that to look 'right' it ought to be as wide as or wider than the window.

Madre Perla is a gorgeous stone - it's going to look beautiful and I think you will appreciate having a decent-sized section displayed vertically there. What color will your walls be?

With the dimensions in the plan, I think it would be lovely if you chose a bs to extend from the open shelves on the left to the tall cabinet on the right. It could be very dramatic. You could combine your Madre Pearla with a mother of pearl tile...

I haven't given much thought to a wall color yet. My kitchen is an L as you undoubtedly figured out from the pictures I posted, with an island. The L cabinets are natural walnut and will have the Madre Perla. The island will be painted and will have a walnut wood top. I need to pick paint colors for both the island and the walls, our house is an open floor plan post and beam. Since it is very open, the colors need to be fairly neutral.
Before I fell in love with the Madre I was thinking about Cambria Torquay, but I kept thinking the kitchen felt too sterile. It turned out it was that I needed something warmer and when I saw the Madre I fell in love. The stone yard actually only had small piece so now we are hunting slabs, but I know they will turn up and I have time. I am a little reluctant to lock in to colors until I have the actual slab, but I do feel that the Madre has a greenish undertone. I may do something like a very light sage for the island and an almond for the walls, but it also has a goldish cast and I could go in that direction too.

I believe we sort of have what you are talking about. Our backsplash continues past the windows, but you can see it is higher under the windows. We also did the sill in granite, which is one of my favorite things in our kitchen.

These pictures during construction probably shows it best. When the window moulding was installed it is even with the end of the backsplash.

EAM44, I love your idea of extending the sink splash to the tall hutch and aligned with the bottom of the open shelves. However, there is a double switchplate to the left of the sink, 9 inches from the edge of the sill and the open shelves are 11 1/4 inches from the edge of the sill. How do you think it would look if it was a curve to the counter? That way I wouldn't bump into the switchplate.

NHBaskets, Yes that is just what I am thinking. Thanks for posting your pics! It looks great. You have a lovely kitchen.

EAM44, I didn't mean to ignore you suggestion of the mother of pearl tile. It is very lovely, and I'll try to take a look at it in person, it might be a little too fancy for me I have a sort of modern farmhouse vibe.

Oh gosh - no worries. I love the granite (it has tons of movement, but the colors are so subtle) almost no one uses it as far as I can tell, and I liked the idea of "Madre Perla" with "Mother of Pearl" (I mean, come on...), but if modern farmhouse is where you're going, that tile's not going with you.

You know what might work for you once you've chosen your slab and wall colors, Fireclay Claymonde sheets. They're rectified ceramic sheets that measure 5 3/4" x 35 1/4" (it's shown in Linen below), or their Crush glass tile in the 2" x 8" size (shown in Icicle gloss below)

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As far as curving away from the switchplate - I honestly don't know. You could just tile around it and choose a coordinating switchplate cover. Then you could keep the lines straight and symmetrical. If you choose glass tile, you can get a clear glass switchplate and paint the back to match your tile. This was suggested by palimpsest on another thread (link below).

Here is a link that might be useful: Outlet Covers for Glass

I really like the Crush tiles, they seem to have a great balance between sheen and crackle. I will have to go check them out.
RE the backsplash, at this point I need to wait & see if there will be enough left of my slabs to do it and take it from there.
Thanks for all your input.