I think the answer has been covered in the previous posts but just as an addition Tom:
If and when you ever get any issues as you have, it would help with the diagnostics if you added a screenprint or snip for illustration purposes, plus the browser and OS (operating system). If you're unsure how to post a snip or screenshot let us know in here and we'll guide you through it.
Further to Craig's post, while in the game and if you are getting sizing problems, you just right click and in the resultant pop-up are Zoom In and Zoom Out. Flick between those two as Craig suggested and your playable screen size will, or should, appear.
One final thought and it maybe of use to others. I use Opera for WGT as having tried just about all the popular browsers over time, I've found Opera is the only one where you can type in the game chatbox while still in full screen mode. All the others request you leave Full Screen mode in order to type do so. Also I have never had screen sizing issues with Opera either. I find it more user friendly than the others.
Whether that's just my system I know not. It also switches from game client to forums easier too.