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 your product name on the top left side of the screen. Under Canvas size, enter your new canvas size.
For performance reasons, the maximum recommended size is 1200x1200px.