You can use your web browser to see the exact dimensions of different images on your site.

Most modern browsers have an option called "Inspect Element" which is accessible by right-clicking over any area of the site—typically a panel comes up showing the HTML content and the dimensions of the highlighted element.

Hover your mouse over an image in the web page, tap the "right-click" mouse button and choose "Inspect Element."

Your browser should display a tooltip with the dimension of the desired image. The first number is the width, while the second is the height.