I am hooked up to a tv via hdmi. I need to shrink the vertical size because I am losing about 10 pixels top and bottom. As that is where toolbars and panels are without their controls I have a very hard time accessing them. I do know that in getting the tv first working I did see sliders that did that job, but as I did not need them at the time I did not touch them but now I can not remember where I saw them. I am using KDE5 as my desktop.
You are looking for 'overscan correction' or 'set underscan'.
Depending on your graphicscard and used drivers you might find this slider in the driver's control GUI (Nvidia-binary has one). A while ago I used xrandr to set underscan in an autostart-script.
Google should find a lot of howtos using xrandr.