Matching the ratio of your photo reference to canvas
One thing that is important to be mindful of, it ensuring that you match the ratio or proportions of your photo reference to your canvas. This can get a bit complicated but as a quick easy option you could get your photo printed at one of the standard sizes below, and then buy the standard canvas size to match it.
8x8 inch photo print - 16x16 inch or 40x40cm canvas (or any square canvas)
6x8 inch photo print - 12x16 inch or 30x40cm canvas (or any canvas in a 3 to 4 ratio)
8x10 inch photo print - 16x20 inch or 40x50cm canvas (or any canvas in a 4 to 5 ratio)
8x11 inch photo print - 16x22 inch or 40x55cm canvas (or any canvas in a 8 to 11 ratio)
8x12 inch photo print - 16x24 inch or 40x60cm canvas (or any canvas in a 2 to 3 ratio)
If you can stick to one of these sizes it will also help in using The Grid Method to assist in your drawing.
For those of you not afraid of a little maths this formula below will help you figure the relationship between the proportions of your photo reference and canvas.
Height of Photo x Width of Canvas = Width of Photo x Height of Canvas