You can resize the Ruler from the edges with the standard Click Drag gesture. On a horizontal ruler you can resize from the left and right sides. For a vertical ruler, you can resize from the top and bottom.

You can also hold the Option key to do symmetric resizing (ie. sizing from both sides at once). This is a standard OSX behavior for all windows.

Fine-tuning with the keyboard

Although mouse-based resizing works great, real fine-tuning can be done with the keyboard. Use the + and - keys to increase and decrease the size of the ruler by 1pt. Hold the Shift key to size in 10pts.

Changing orientation

The ruler can be toggled between horizontal and vertical orientations with the Space key.

Moving the ruler

You can move the ruler with a Click Drag gesture anywhere on the window. As you move between monitors, the tick marks will auto-adjust for retina and non-retina displays.

You can also use the keys to move the ruler by 1pt in any of the 4 directions. Hold the Shift key to move by 10pts.

Relative Position

As you move your mouse along the ruler, you will see the relative position within the ruler. The relative position is shown next to the ruler size.

This is useful to get a rough measure along a side, without having to resize the ruler.