Indoor Trompe l'Oeil is usually painted on a canvas subsequently fixed in place. Of course, paint can also be applied directly to the wall, but you may loose the decoration in case of water damage, and you loose it if you move house...
To insure that a proper illusion is created, Trompe l'Oeil requires some architectural frame opening onto the scene behind. Thus, perspective provides depth.
So, Trompe l'Oeil comes traditionally in various sizes :
Images of windows (roughly 2ft7' x 3ft11') are easy to put up in bathrooms, kitchens or bedrooms. They can also come in sets for a living room or study.
Images of doors (around 4ft3' x 5ft3') open up the end of a corridor or hallway.
Images of double doors measure often 5ft3' x 7ft6', but they may reach 6ft6' x 8ft2' or more.
Beyond these sizes, panoramics. do not require architectural frames. However, they are best suited to larger walls.