Each portrait ends up a custom shape to best suit the original photo.
That way your portrait or your horse is always as large as I can make it with minimum background.
If I used standard sizes like many artist’s you may have a rectangular shape canvas but your photo cropped is square so you end up with a small horse and a lot of background.
Although this is a dog portrait the concept is the same.
By making the canvas a square custom shape Shirts face ( the important thing)
is much larger and there is not much background.