Some artists have the ability to stretch their own canvases and create custom sizes. Cool for them, but I do not! I typically use the most common, standard sizes of gallery wrapped stretched canvas, especially for Commissions, that are generally available in local art supply stores or online. This is the way I divvy canvas sizes, which means for me - how I can get it in my car, get it home (I have a small studio), and even ship it. Rule of thumb, the bigger it is, the harder it is to handle it and the more expensive to ship it. For larger sized artwork, an option is to break the artwork into diptychs or triptychs.