You’re looking at a web page and see the size of the image on the page is what you want. What is that size? How do you calculate it? Hold a ruler up to the screen? Cut and paste into a graphics tool like IrfanView? Nope. Get yourself a copy of the very handy Cool Ruler.
Cool Ruler stays on top; measures vertically or horizontally; in CM, IN, or PX with an option to customize measurement; and includes a calculator. It takes up virtually no memory or hard drive space, and is a handy tool to have in your digital tool box.