When you want to edit the shape of a layer or selected pixels, one very effective way to do this is with Free Transform. Either press Command-T (PC: Control-T) or use Edit>Free Transform to get the transformation handles. Then you can click-and-drag on any of the handles, or use the settings in the Options Bar to resize numerically. (Use the default measurement of percentage, or Control-click [PC: Right-click] in the measurement field to change to pixels, inches, or whatever unit of measurement you want to use.)
Here are some important keyboard modifiers that change the way Free Transform operates:
Shift: Keeps the transformation proportional.
Option (PC: Alt): Transforms from the center outwards.
Command (PC: Control): Transforms the one handle you click on.
Command-Shift (PC: Control-Shift): Skews selection.
Command-Option-Shift (PC: Control-Alt-Shift): Creates perspective.
Move your mouse just outside the corner handles to get the Rotate cursor.
Once you're finished transforming, press Return (PC: Enter) to "confirm" the transformation. Press Escape to cancel out of Free Transform.