You might be using a small canvas size.
Please tap the (...) button > Canvas Size > Custom, to check and change the project size.
To reach optimal resolution, you'll need to keep the existing ratio between height and width. To do so, multiply both of them by the same number. Your project can reach up to 5000x5000 pixels.
For example, if the dimensions you require are 500X200, you can multiply both by 10 (or any other number), reaching 5000x2000 pixels.