First is where you anchor. If you have a low Olympic anchor, you need a long sight window. If you have a index finger Corner of mouth and shooting field you need a long sight window to cover the distance from 80 to 7 yards. If you are shooting 3D or hunting only out to 40 yards a 4" sight window would most likely cover that distance.
If you have a high cheek bone anchor you can get by with a sight window three fingers wide for 3D and hunting.
There are shooters that gap off the riser. I won't go into to that now but it also is also a consideration
Guys will use their riser for aiming. So you could have a custom riser built so that with an arrow of say 190 FPS coupled with your anchor the top of the sight window would be point on at 15. Then by shaping the edge of the window as it comes down you could creat other references for other yardages.
You would figure this by taking a metal riser and glueing foam to the site window and forming it. A good bowyer could then copy that riser and sight window.