Some months I ago, I purchased 2 canvas of unusual format: one was 100x80 cm (i.e., almost square), the other one was 116x65 cm (i.e. like cinemascope-format).
When painting, I do not plan very much, I just wait for inspiration. The inspiration for the cinemascope canvas came fast; I wanted to do a reprise of my "Jungle animals" motive you can see the result here: http://www.pbase.com/etlea/image/36434767
But concerning the "square" canvas, I had to wait much longer.
In Venice, I saw an old painting showing a venetian mask and some floating thing. I did not take a photo of the picture, but when I remembered this, voila - I had the motive for my "square" canvas.