How do I take a screenshot with dimensions?
Shutter
Shutter is a feature-rich screenshot program. You can take a screenshot of a specific area, window, your whole screen, or even of a website - apply different effects to it, draw on it to highlight points, and then upload to an image hosting site, all within one window.
You can set the size of default selection in it's Preferences. Go to Edit > Preferences > Advanced and change the value of start with selection size of.
You can get more details about Shutter on it's website.