Screenshot a specific dimension, by default 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.

thanks
A popular capture tool that I'm aware of is SnagIt.   SnagIt does have an option for capturing a fixed region (pre defined size).

SnagIt isn't the cheapest of the capture tools out there...   just a popular one (and I also use it frequently for work).