Page Ruler Google Extension

Draw a ruler to get pixel dimensions and positioning, and measure elements on any web page. Page Ruler lets your draw out a ruler to any page and displays the width, height and position of it.

Some features are that you can draw a ruler to any page and view the width, height and top, bottom, left and right position, drag the edges of the ruler to resize it, use the arrow keys to move and resize the ruler, show guides extending from the ruler edges, manually update the size and position of the ruler from the toolbar to make precision changes, enable "Element Mode" to outline elements on the page as you move your mouse over them, navigate through parents, children and sibling elements of any measured element.

This extension is easy to use, and it does what you want it to do. Would recommend to others would are curious to use this.

Download here

More info