Dimensions
Easily determine sizing and spacing using the dimensions tool. This tool finds the nearest edge in all directions from the mouse cursor and can quickly let you measure the spacing between graphics. With adjustable tolerances and smart handling of gradients the dimensions tool works over a wide variety of graphics.
How to use
There are three types of the dimensions tool- the crosshairs, horizontal only and vertical only. The tools all work the same the only difference is that for some uses you may only want information on one axis.
Click the crosshairs to start the dimensions tool and as you move your mouse around it will constantly measure the distance between visual elements in both directions.
Since the tool works using the on-screen graphics it can function on any screen graphics, it also has logic to handle gradients.
The arrow keys will move the mouse one pixel in any direction and may be used for precise positioning of the mouse cursor.
Tolerances
The tolerances used to find the edges can be tweaked as well to either ignore or pick up more subtle changes. This is accessible using the mouse wheel or in the preferences.
Tolerances can be adjusted to handle more complex graphics.
Redlining
Multiple measurement adorners can be added to the screen by pressing the 'Enter' key when the adorner is where you'd like it. This way it's easy to add multiple measurements.
Individual adorners can be closed via their close
button 
or by pressing the 'Escape' key to close all of them.
Once the measurements are complete it's easy to use a screen capture utility to save them.