Max resolution on recent devices (e.g. iPhone 6, 6 Plus, 5+, iPad Air) is 4096x4096. On older devices (e.g. iPhone 4s, iPad 2, etc.) the max resolution is 2048x2048.
However, even if you are on a recent device, when memory is low, the app will downsize to 2048 to prevent app crashes. Currently, the user doesn't know when the app is in low memory mode... something we will probably make changes to in the future. In addition, if the app crashes while already in low memory mode, the app remembers and stays in low memory mode the next time it starts.
While we support 4096x4096, there will be times the app downsizes without you knowing.