A layer has not always the same size as the image. It can be smaller or larger, in which case some parts are hidden. Selecting this item will activate a dialog in which you can set parameters for the active layer dimensions. This command changes layer dimensions, but does not scale its content.
When you call this dialog, active layer original dimensions are displayed. You can change them. For this, you have two input boxes which are linked if the associated Chain symbol is not broken: then Width/Height ratio is kept; else, both dimensions are independent.
Default unit of measurement is pixel. You can change it by using the drop-down list: for instance, you can use a percentage W/H ratio.
By default, the resized layer is placed in the upper left corner of the image. Here you can set the offset of the upper left corner of the layer from the image left corner, in both dimensions. Default unit of measurement is pixel; you can change it by using the drop-down list. You can also place the layer at the center of the image by using the
button.