Here are the maximum canvas sizes by device in pixel: For example, on the 12.9' and 11' M1 iPad Pros (updated to iPadOS 15 or later), you can make any canvas provided neither side is longer than 16,384 pixels and the total size of the canvas doesn't exceed 134 megapixels. You can create a canvas of any size and ratio in Procreate as long as it doesn't exceed the width/height limit for your particular device. The iPad (5th Generation), iPad Air 2 and iPad mini 4 can create canvases up to 8,192 pixels in either direction, and for the iPad Pro models this limit is increased to 16,384 pixels. The other limit is on any single dimension - width or height. For example, the 12.9' and 11' M2 and M1 iPad Pros and the iPad Air 5 can create a canvas up to 134,217,728 pixels total - around 134 megapixels. The limit on total canvas size can be found by multiplying the two dimensions of any of the below maximum sizes. There are actually two limits on canvas sizes in Procreate, both imposed by the hardware of the iPad model you're using.
