This question already has an answer here:
I would like to do a screenshot/grab of a specific size, is there a way to set this on Yosemite, or is anyone aware of a software tool/app to do this?
I currently have to draw the shape, but often start at the wrong place so have to go through the motions two or three times, when if I had a fixed size i could position it over the area to screenshot to make sure it is correct.
Unlike the related question, The exact position on the screen may differ as I'll be taking a screenshot of my own web site but the browser window might differ in its location.
I am open to all possibilities whether programmatically, workflow, app or website.
On Windows there is a tool called MWSnap that allows settings a default fixed area.