Since my art is non-representational, also called "non-objective," I find that the square form fits it best, and this is the most common size canvas I use.
Abstract art also lends itself to large images, so I tend not to paint on canvases smaller than 24" x 24".
My abstract art was influenced early on by the abstract expressionists, but I find every visit to a gallery or art museum to be inspirational even though many art movements have come along since then. I am most interested in art that is expressive in terms of color, form, composition, texture, and energy. I love the first visual impact a great piece of art has on me as I approach it. Then gradually I absorb the details as I look more closely.
I'd have to say that painting is my favorite art form, especially expressive painting. It's the two-dimensional surface that I love, and I find it harder to relate to sculpture, photography, video, or installation art.
Some artists find it easy to move from one medium to another: painting to sculpture, for example. But I fell in love with painting early in my art education and stayed there. I do remember making a head out of clay, and enjoying a pottery class once, but art is long and life is short (originally from Hippocrates in Greek).
Painting offers a lifetime of exploration, discovery, and joy as far as I'm concerned.