The canvas size is infinite.
However, you can create a frame, so you can draw for a certain resolution, using the "Edit Frame" toolbar icon (or via menu in
File >> Edit Frame), setting the desired size and enabling it.
You can still draw outside the frame, the view will just be obscured.
When you use
File >> Export, while the frame is enabled, it will export the image inside the frame. If you disable the frame, it will export the area on which there is paint.
The current FAQ doesn't have an easy to find wording, instead it answers "How do I know how large my image is?" with
Starting with version 1.1.0, MyPaint features a frame tool which
allows you to control how big your document is, and this has a "Crop
to Document" command. The image size can be adjusted in pixels, mm, or
The canvas is still infinite, however.
without pointing out where this tool is.