If you're building, why not bump out the window? You can go out just a few inches, or far enough to have slim side windows as well.

The depth of the stool also depends on the type of window. Vinyl is shallow; clad wood is deeper.

If the plan can accommodate it, I often pull the cabinets out 6 in. and put a shelf on top of the backsplash the full width of the room.

Tell us all of the parameters and post a plan.

Here is a link that might be useful: [here is a link to the URL you posted[(https://www.houzz.com/photos/spring-parade-2012-eclectic-kitchen-grand-rapids-phvw-vp~1073279)

You can't get a practical wider window stool unless you do the little bump-out.

What you want is called a "Box Bay Window" - a square bay window. It's ofetn used where you need or want a deeper sill.

http://www.williamstanleyco.com/boxbay.html