Can you take a picture from the outside. So the sun is not shining thru? It might be easier to figure out without the glow??

I agree with QuiltinNan. Break it down into doable sections and connect them

I like Thimblebugs technique! And remember-it's ART. You don't have to match the window exactly.

If there is any way to get a "straight on" photo of the window, you could have it printed onto fabric at http://www.spoonflower.com and use it for reverse applique.

You could do a reverse applique with hand stitching. This a technique used in the "Molas" made in Central America. As for enlarging the pattern, I don't know unless you have a printer that you could cut the picture in smaller pieces and then print them enlarged on your printer. Or get an artist friend to grid the picture and draw it on a larger grid to get the larger size. Batik or hand dye fabric would give you the effect of light shining through.