Canvas size what ratio to use in digital painting. Here is an in depth detail on how to set up your canvas size in Photoshop.
Digital art tips canvas size what ratio to use in Photoshop
Common ratio for painting in Photoshop are below in inches. If you are using the Metric system, you will have to convert from inches to centimeter. 1 inch = 2.54 centimeter.
5 x 7 inches 300dpi
8 x 10 inches 300dpi
9 x 12 inches 300 dpi
11 x 17 inches 300 dpi
I wrote an article on it a while back with more detail you can go to what size to paint in Photoshop for illustration.
Watch a video tutorial on canvas size and ration that I recommended in digital painting.