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