Going to resolution 8 helps a little bit. Your model is upscaled to fit the new resolution, and allow for slightly finer details. Using the Color Blur tool may help to hide the jaggedness. If that doesn’t work for you, maybe using Brush tools will get you the finer detail you’re looking for? It’s found under Sculpt (on the far left side of the menu).
If you model does not fill the cube, it can be scaled up to fit. You can use the selection tools to select your sculpture, and then cut and paste it at a larger size.
In future updates, we may allow for higher resolutions. Then you’d be able to create finer details with the paint tool.
Hope that helps!