The Layer Boundary Size lets you set the the boundary of the layer. Remember you can have a smaller or larger layer than the image boundary size. When you enlarge the boundary size, then you will add some space to paint on to your layer. Naturally it will be vice versa when you make the boundary size smaller.
The Size area will control how big or small your new layer boundary will be just as when you created a New Layer. The Offset area will control how your layer is clipped or expanded.
The Offset fields will control where your upper left corner of the old layer will be in the "new" layer. The best way to control the new location and how and where the layer will be clipped is to drag the "layer preview" in the Offset area to the right position. Make the final adjustment with the spin buttons (or type it in by hand) if it needs to be pixel-exact. The thin outline is the new layer size and you have to drag the old layer to the right position so it will be clipped according to your demands. If you make the layer larger then the outline/canvas is the new layer size and you have to drag the old layer to the right position within it.
If you uncheck the chain in the "Size" area, you will be able to have different ratios when you make the "new" layer . It's therefore possible e.g. to have a layer which is smaller in X direction and larger in Y direction than before you altered the layer boundary size.