You can use a picture frame as large as you would like a mirror, with the glass still in it and buy the spray for the back of the glass in the frame. Just spray it and follow the instructions on the can. You can get mirror backing spray at the local home improvement stores. Hope this helps Toni, it sounds like fun as you can be as creative as you like (for example use a window or window with panes, or any shape picture frame/glass depending on where you want the mirror. The paned window one would look great at the end of a hall or top of the stairs with a table under it for example.
If you are just looking for a mirror effect the mirror sprays woek perfect. But not if you want to see your true reflection. The spray tends to distort your features.