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.