The canvas size is the size of your product preview. It is also the size of all of your product images.
When uploading your first image, your canvas will be automatically resized to the size of this image.
To resize your canvas, log in to your MyCustomizer dashboard, go to your product page and click on BUILD. Click on the Settings icon and under Canvas size, enter your new canvas size.
If you previously uploaded images that have a different size than your canvas, you will need to re-upload them so that their size is the same as your new canvas size.
The images you need to re-upload will be marked with this icon :