Page Grid Ruler

Features:
Displays a grid view (custom gride size)
Displays a reference box
Displays a ruler with width, height, start, and endpoints

How to Use It:
On a page, press the action button. The extension switches the view to grid mode. In this mode, you can draw a custom ruler. This ruler displays the live width, height, start, and endpoint values.
Press the "Escape" key or press the action button once to switch back to the normal browsing mode.

Does this extension support "mm", "cm", or "in" units?
No, there is no way to get accurate values. So this extension uses the pixel unit. For converting the pixel unit to "cm" or "in", the PPI (pixel density or resolution of a computer screen) value is needed.

How can I change the default grid size?
Right-click on the action button. There are two menus. You can change the grid size and the grid thickness from these menus.