improve 1) hotkey setting; 2) description of what parameter D is; 3) turn it off with the Esc button; 4) reduce the font size.

hard to see the numbers. no customization

Why I can not move created rectangle?

good

It's hard to get pixel perfect. It's hard to see the numbers. The numbers are in the wrong spot, often in the way. I had to uninstall to look for something else. sorry.

Umm, so what is this measuring in? px? cm? Can't tell and it ain't sayin'. Replaced with Measure-it.

Consider allowing to increase the width & height after are created the square or rectangle, you can not create form first select the exact size with the mouse (the edges to become editable when you put the mouse over the edges to show an double arrow which to allow to increase or decrease the size and add checkmark who stop the edit and after to can move on the screen your created figure).
Display the unit of measurement, is it pixel, cm or inch?
To can save as predefined figure to can load in other tab/window!
Scenario: A customer tells you that the distance between the title and the photo is not exactly the same as on his competitor's website and he wants the same distance. If you can save a figure created as a predefined (by giving it a name), you can load the same figure on the other tab/window and to show him if is exactly or not, and you do not have to recreate it again.
To can generate measurable figures in units, for example: create a ruler of 24 pixels width and 4.5 pixels of height.

very useful. add please hotkeys for open and close

it doesn't work for RTL sites well.

Work as its name. I wish this extension can change background color of selected area to be easier to read