I think you are on the right track because uppers would probably highlight the asymmetry of the window, but I do think you need something to take up some of the empty space on both or at least the larger side, if it's a fair amount of space. Would you consider floating shelves?

Also, please post a layout so I can be sure I'm envisioning this correctly :)

The size sounds lovely. I do have one suggestion. I have a large window above my sink, and it seems it would be really difficult to open and close it if it were a single hung since you have to reach across the sink/counter to reach it.

Ours is a combination of a fixed non-opening window in the center, flanked by two casements. This provides enough ventilation for us, but I suppose you could also do three casements. They should be able to match the panes or frames so the windows look consistent with your breakfast room window even if they open differently. Just an idea!

Honestly, I'd work on your layout first so you'll know the ideal placement for your window, especially since it appears centering your sink under the window is important to you. For example, how do you know your sink will be best centered where the window currently is? Maybe it would be best shifted to the left (or right) and you would probably want to shift the window the same to keep it centered.

Although, you don't have to center the sink under the window...that's a design hold over from before there were DWs and people spent a significant amount of time in front of the window washing & drying dishes. Today, the vast majority of time spent in the kitchen is spent prepping (70%). Followed by a very distant second of cleaning up (20%). Finally, cooking (10%).

I'm only suggesting you wait b/c often a kitchen layout drives the windows and, sometimes, interior walls. It's very frustrating to us when someone asks for layout help and then they tell us "but you can't move the sink" or "nothing can change". The less flexible you are, the more difficult it can be to get a good, functional layout (unless you're lucky and everything "falls into place"...it does happen occasionally, but not always.)

While you're considering how it would look from the inside, please spare some consideration about how it will work on the outside. These days I'm seeing lots of windows just poked through willynilly without thought about the pattern and rhythm of the building's overall fenestration. Windows are substantial elements of how a building looks and having an over-sized or unbalanced one can really effect how attractive the house appears.

If you can, take a pic of the proposed elevation and photoshop in the larger window before making the change order.

Buehl's point about considering the layout before commiting to a larger window (and a 72" wide window is a large window) is important, too.

L