Lately this question has been coming up a little more often from clients. I don't know if recently it has become easier for people to accidentally zoom in on their browser window or perhaps people are clicking on the zoom control some mouse's now have; but either way here is how to get your browser back to where you want it.
For Mozilla Firefox:
You can either select Ctrl + 0 while the browser window is open or follow the instructions below.
Go to View > Zoom > Reset
For Internet Explorer
You can either select Ctrl + 0 while the browser window is open or follow the instructions below.
Go to View > Zoom > 100%
