Resize
There are a few different ways to resize a layer. You can change the layer size in the properties window for the layer. And you can use the numpad-hotkeys. Or you can use this function. You can resize a layer in all 4 directions at once. You add tiles by using positive numbers and remove tiles by using negative numbers.

If you've previously selected an area in the editor, you can also press the 'Crop layer to selection' button to crop the entire layer to the size of the selection.