What I have done, just to keep things simple, and prevent errors between design and production, is first consider working at full scale. Think about it. Illustrator max artboard it 227". Your current project fits well inside that right? Maybe cumbersome, but you can make a new doc for each window. Then if you need to proof the design via PDF, you can a just keep all your details/notes in within a box that is the same ratio as say an 8.5x11 or 11x17, and, print to a PDF and choose "fit to page" from the print dialog. Simple PDF proof from your art.
Another option, which works really well for my workflow, is just working at 10% scale. Simple. No math to remember. When setting your document/art board size, keep "constrain width" locked, and move the decimal point. For example: 96" x 36" = 9.6" x 3.6". Now you have more room to work if there are many windows/signs you need to build. Just make sure to note the scale some where for your production guy/gal. If that is you too, then you are good to go. Now all the above only apply if your art is purely vector. If you are adding raster/bitmap elements you will need to account for a few other variables: Viewing distance and final output resolution. Regarding viewing distance, think about how far your end user will actually be looking a the product. Second story from across the fwy? Or from 2' as they walk past it in the entry? Regarding final output resolution, 150dpi at full size typically is more than enough pixels to get the job done. I'm sure there will be some opinions opposite of all this. YMMV.
Probably way (waaay) more info then you needed. Hope some if it is useful.