http://www.forrestrace.com/shapes-sizes/
How and Why do you think painters pick the canvas shapes and sizes they pick?
For example, rectangular, square, triptych? Is it based on the composition? For example, John Singer Sargent got a lot of flack for “The Daughters of Edward Darley Boit” because he chose a perfectly square canvas. Do you think it really matters? Thank you. 
Artist’s paint their feelings and what lies within their heart and soul, they paint because it is what they love and are driven to do.
This affects not just the subject but the materials and medium they use also.
Imaginaria – 06 All Shapes and Sizes (Short Circutz)