Das Werkstatt

[PROBLEM]
Example:
In an image 200x200 px, you can only draw in the upper left corner, but not in the whole canvas.

[SOLUTION]
Each layer has its own size.
Turn the layer border visible by selecting
"View > Show Layer Boundary".

You should now see a yellow/black dotted line indicating the layer's boundary. You can resize this by choosing either
"Layer > Layer Boundary Size" or
"Layer > Layer to Image Size"

[PROBLEM]
You cannot paint in transparent parts of a layer.

[SOLUTION]
In the "Layers, Channels, Paths, Undo" toolbox you can choose to "Keep transparency" of each layer by enabling the checkbox right of the "Mode" dropdown box above the layer list.

Uncheck this option and you should be able to draw normally.