I haven't found this problem with the live CD.
Firstly, I opened PC Control after putting in the required root password
Under Hardware > Configure Graphics > Configure Video Card, I changed my monitor to Flat Panel 1366 X 768 (my laptop)
Next line down, I changed Resolution to Other > 1366 X 768. Accepted the changes
After accepting this, I logged out and then back in again and I had the full screen.
If the panel is not full width, you can Unlock the Widgets > Panel settings > More Settings > Maximize Panel.
Now that last line certainly applies for the KDE version. It's a while since I messed around with LXDE but there should be a way of changing the panel width.
In the Open Box version, right clicking on the panel allows one to increase the panel width to 100%. It just might be similar in LXDE. But I have no doubt you will get it fixed.