The eight handles don’t just indicate selection; you can also use them to resize a layer. Grab any of the handles and drag them to resize layer. If you also hold down the Shift key (⇧) it will resize the layer proportionally; keeping width and height in the same ratio.
To resize just the width or height of a layer, grab the resize handles on the sides of the layer, to resize in both dimensions, use the corners.
If you hold down alt (⌥) it will resize from the middle out instead of from the opposite corner:
Resizing using the Inspector
You can also select a layer and type its new dimensions in the Inspector. If the lock icon is closed, its proportions will be maintained when scaling. Also, you can use these shorthands to scale from an anchor point other than the top-left corner:
- l: scale from the left (Default)
- r: scale from the right
- t: scale from the top (Default)
- b: scale from the bottom
- c/m: scale from the center / middle
So, to scale a layer so that it's 50 pixels wide, from the right, you'd set its width to 50r.
You can also resize layers using the keyboard. For very pixel-precise adjustments, this is usually easier than using the mouse. To do so, hold down the ⌘ key and use the arrow keys on the keyboard. ⌘ → will increase the width of the layer by 1px just as ⌘ ← will decrease it by one. Similarly, ⌘ ↓ and ⌘ ↑ will increase and decrease the height by 1px respectively. Holding down the shift key changes the 1px increment to 10px instead.
When you resize a layer, its stylistic elements will not scale up or down accordingly; a 5px stroke on a 50x50 shape will stay a 5px stroke when the layer is resized to 150x150. To resize a layer and its properties (corner radius, effect, etc) all in one go, use the Edit › Scale menu item instead: