By the time you get a window in between 2 studs, 16"-1 1/2"=14 1/2", you will still need at the very least 1/2" of space around the frame to plumb, level and square the frame. 14 1/2"-1"=13 1/2". Then the frame, 3/4" on each side, 13 1/2"-1 1/2"-12". Then substract the width of the sash 1 3/4" on each side of the window, 12"-3 1/2"=8 1/2" wide glass. The frame and the trim adds up to be as wide as the glass. Makes an odd looking window.
The windows can be made. You might consider a casement window. The sash is sometimes more narrow. It may give you more glass. Many people do not care for the way they 'crank open'. They have been known to fail. Good manufacturers can provide replacement parts.
All of that said, why don't you just brace up and put in a full width window?
It would look far better. wouldn't be as bad as you think. the worst part would be getting the header in and the drywall repairs to cover it. Although I have seen valances built out over the windows, from the ceiling down. That would eliminate the mud and sanding. Just a rough, but flush tape bedding, would prep the patch. Then build the valance with 1/2" cabinet grade plywood. instal the casing, a little crown moulding around the top. Sand the edges smooth and prime and paint.
Just ideas.. disregard if you like
Do keep us informed.