I'm a contributor to Tiled - Map Editor. It's a tile-based map editor for developing games. Currently it supports various types of maps (isometric, hexagonal, orthogonal, etc.) but of fixed sizes. Obviously the map can be resized to some other size, but it would be great to have a feature similar to MyPaint in Tiled as well. You can look over the issue ticket where this feature has been requested, Supporting Infinite Maps.
I'll be working on this feature and before starting, I'd like to know how exactly does MyPaint support infinite maps, so that it helps towards the development of Tiled.