The Crop Tool allows you to interactively change the image bounds.
Simply drag an edge or a corner of the canvas to resize.
The "Group All Layers" button creates a new layer group form all current image layers. This can be used to prevent cropping from affecting the result of Interactive Filter Layers. (The results of some filters depend on the image bounds.)
The "Fit Canvas To Layers" button adjusts the image bounds so that all layers are fully visible.