Using Masks and Layers—Painting, Filling, and Color Tools - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 218

Transformations—Scaling a Layer
In previous exercises you scaled entire images, changing their size, using the
Image > Scale Image menu item. In this exercise, you will scale a single layer
and transform it.
Take a look at your image. You will see the hazysky layer in the image
window. The largest part of the layer is hidden behind the landscape layer,
but if you set the hazysky layer to active in the Layers dialog, you will see its
contour, indicated by a black-and-yellow dashed line. You will now freely
transform this layer and it it into the image. Make sure it is activated in the
Layers dialog.
Alternatively, you can enter numerical values by choosing Layer > Scale
Layer . This is the preferred method when changing a layer to a size that has
already been numerically defined. However, for this exercise, it is preferable
to use the mouse because you will be scaling to fit the layer aesthetically
within the image. The Scale tool from the Toolbox provides you with options
necessary to do this. You can also find these options by choosing Tools >
Transform Tools > Scale .
Figure 3.43
Scaling a layer