Element Size Measurement

After clicking on the extension icon in the top right, it will activate.
Then, when you hover over any element on the screen a little popup will display:
Width in px
Hight in px
Area - Total area of the block you hovering over in px squared, and total percentage this block would take up of your screen (it can go above 100% which means more than one screen)
Visible Area - Area taken up in the viewport (your browser screen) in px squared and in percent of the viewport

Note: It doesn't work on absolutely all pages just yet, pages like settings and some google account pages and such require additional permissions, hence - doesn't work on those.