Custom Canvas window. Source: iMore Here you will be able to select the dimensions in millimeters, centimeters, inches, or pixels. The dimensions and your hardware determines the number of layers that you'll have available in your project. The smaller the dimensions and the more powerful of an iPad you have, the more layers you'll have at your disposal. Unless you're doing a very complex piece, you probably won't need hundreds of layers. However, if you plan on doing larger artwork with a lot of layers, you may want to consider getting an 12.9-inch iPad Pro since it'll give you a larger surface area and better hardware specifications.