MeasureIt!
Draw out a ruler that will help you get the pixel width and height of any elements on a webpage.
https://chrome.google.com/webstore/detail/aonjhmdcgbgikgjapjckfkefpphjpgma

Awesome Screenshot: Capture & Annotate
Capture the whole page or any portion, annotate it with rectangles, circles, arrows, lines and text, blur sensitive info, one-click upload to share. Support PNG and shortcuts. 
https://chrome.google.com/webstore/detail/alelhddbbhepgpmgidjdcjakblofbmce