Will scale the layer content, and also make the layer boundary either size smaller or larger. The difference between "Scale Layer" and Set Layer Boundary Size is that "Set Layer Boundary Size" will enable you to have a smaller or bigger layer without scaling the layer content (i.e. it will only add some space to on around the layer or clip the layer).
"Scale Layer" will only scale the currently active layer. Scale Image will scale all layers in the image.
You can set the new layer size either by altering the size or ratio. You have have exactly the same options as in the New Layer dialog.