Import this layer first.
Next, import another image.
Layers > Import from File...
If it is oversize, the canvas will expand to accommodate it. Let it do so.
Make a Rectangle selection around the part of that layer that you want to use.
Copy/paste into a new layer.
Ctrl + C
Ctrl + Shift + V
Delete the original oversize layer that you have just copy/pasted from.
The selection you copy/pasted should still be selected,
and the Move Selected Pixels tool will be active.
Re-size using the Move Selected Pixels tool on a corner adjusting nub.
Repeat for each additional layer.