There are actually two limits on canvas sizes in Procreate, both imposed by the hardware of the iPad model you're using.
The limit on total canvas size can be found by multiplying the two dimensions of any of the below maximum sizes. For example, the 12.9" and 11" M1 iPad Pros can create a canvas up to 134,217,728 pixels total - around 134 megapixels.
The other limit is on any single dimension - width or height. 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. 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. For example, on the 12.9" and 11" M1 iPad Pros (updated to iPadOS 15), 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.
Here are the maximum canvas sizes by device in pixel:
- iPad Pro 12.9", 11" M1 on iPadOS 15: 16384 x 8192, 11585 x 11585
- iPad Pro 12.9", 11", and 10.5"; iPad Air (4th generation), iPad mini (6th generation on iPadOS 15) - 16,384 x 4,096, 8,192 x 8,192
- iPad Pro 9.7", iPad (6th generation) and iPad (7th generation), iPad mini (5th generation), iPad Air (3rd generation) - 8,192 x 4,096, or 16,384 x 2,048
- iPad (8th generation and 9th generation) - 7327 x 7327
[From Procreate FAQ]