Right-click on a panel. Select Panel Preferences. Pick the panel that you are wishing to change the settings for. Uncheck "Don't reserve space on borders."
That should do what you are wanting to do.
Better still (IMO), do the above and then also check "Lock panel" and "Automatically show and hide the panel." The former will remove those "handles" that allow one to grab the panel and move it around (something most of us probably don't do on a daily basis anyway) but take up extra space. The latter will make the panel appear when you move the mouse cursor to the screen border and disappear when you are finished using it - which means maximized application windows actually use the entire screen.
In addition to allowing you to use the entire screen, making the panels autohide means that you can increase their size without worrying about losing screen real-estate. I like to set my panels to a size of 40 pixels or thereabouts.
Mint 17.1 Xfce 4.12.
If guns kill people, then pencils misspell words, cars make people drive drunk, and spoons made Rosie O'Donnell fat.