To resize a layer, make sure to select the layer with the Select tool (v) first.
When the layer is selected, it will show a dashed rectangle around it, along with 8 resize handles and 1 rotation handle above.
Click and hold one of the 8 resize handles. While holding, move the mouse to get to the desired size. Release the button to complete resizing.
For a better control, use the keyboard shortcuts as you resize.
Hold the Alt key to resize from the center of the layer.
Hold the Ctrl key to resize along the grid.
Hold the Shift key to preserve the layer’s aspect ratio.
Ctrl / Alt / Shift keys can be used in combination to combine the effects together at once.