How can I measure PSD layer sizes and distances in pixels (px)?
Go to Preferences > Units & rules > Units > Rulers: [dropdown-list] to make sure you have Units set as pixels (px).
In Photoshop CC 2014 you can use smart guides to measure distances. Take the Move tool, select your layer and press CMD (macOS) or CTRL (Windows). Then you can just point at things and see the distance. The same thing is also shown if you move objects while pressing CMD (macOS) or CTRL (Windows) or if you duplicate by dragging and pressing CMD + Alt + SHIFT (macOS) or CTRL + ALT + SHIFT (Windows).
In photoshop CC and newer there is the Transformation values infobox. For example, you can use the Rectangular Marguee tool to draw and area to quickly get a distance. Use this sometimes to get distances/dimensions if you don't need exact dimensions. It should be activated on default. If not, go to Preferences > Interface > Show transformation values: [dropdown-list].
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