AdoramaPix canvas prints offer you the ultimate flexibility by allowing you to choose any custom size up to 29 inches (73cm) tall and 40 inches (101cm) wide.
In the setup menu for your canvas print, below the default sizes you will see a panel with two input boxes (shown below with the numbers 8 in them). This will allow you to set any height and width for your canvas print - up to the maximum simply by clicking in the boxes with your mouse and entering a new number.
If you wish to maintain the same aspect ratio (proportions) of your image, then click on the lock icon indicated below (left). When this option is ON you will see the icon change to a closed lock (below right) and whenever you change one size value, the other will change automatically to maintain the same proportions.
Note that locking the aspect ratio will maintain the aspect ratio you had, and not the aspect ratio of the original photo. i.e. if you have custom settings of 8 height and 10 width and you lock the aspect ratio, and then change your width to 20, the height will change to 16 - even if the original aspect ratio of the photo was 2:3.