This sounds like a bad idea. Windows typically get much of their strength from being shimmed and nailed into a wall; by themselves they don't have the structural integrity required of a door. If you try to support it from one edge (by hinges) it will probably be pulled apart by its own weight.

What jon stated is quite right. It's much too lacking in structure on its own. You could however make a mortise and tenon-framed door and set the sash into it, but it would end up even bigger. Let's say, 9'1"x 5'10". Think of the hinges needed to support a monster like that!
Casey

Okay, thanks for your input. So, what if I can add some support on the bottom? Say some small wheels that sit on the floor and let it roll open. Despite the large size of the window, it did not flex at all when we moved it from one house to the other in the truck. I has a very sturdy frame.

Was the window originally built to open, and if so, how was it hinged? The proposed use may not be that different from the original. Perhaps you could check the frame integrity and reinforce any points you think will be stressed in novel ways.

Having said that, it may be tempting fate to let it hang all the time hanging from one side (even if you use long hinges that extend across the top and bottom - Lee Valley has some). It seems to me it should definitely be supported when closed. If you can support it when open, even just on a stack of books temporarily placed for the purpose, so much the better it would seem to me.

Another issue is whether the wall unit will be stable with the weight of the window on the front. I've had a narrow cabinet that was badly designed, unless loaded it fell forward when the plywood doors were open. It would strike me as prudent to fasten the bookcase securely to the wall.

Or, is it feasible to make the door sliding rather than hinging?

KarinL

Thanks for your ideas Karin. I thought about the sliding track, but don't really want to see the track when the "door" is closed and the track would definitely need to be longer than the wall unit. I never saw the window when it was installed. I got it after it was removed. It was just a stationary window, not one that opened. I think I might investigate the wheel idea a little more. I also think I would have to support the opening side even when it is closed. I plan on attaching the wall unit to the wall because I know it could fall when the door was opened. I have 2 of these windows and am trying to maximize storage in my new little cottage. Thanks again!