Gimp beginner's guide - Alter the width of your tool box and layers panels
for a quick look at the single window mode check out • gimp 2.8 single window mode workspace
To alter your tool box panel width --
Hover your curser over the line between the tool options panel and image window.
When your curser turns into a drag handles, drag them in or out, with your left mouse button.
To alter the layers panel --
hover over the line between the image window and the layers panel, and dragging it's drag handles in or out.