Not possible, unless you mean scaling. Then, on Windows, create a shortcut to the .exe, Right Click > Properties > add argument "scale:2" to the end of the textbox.
My counter reasons: Its already half implemented, but in a windowed mode via command line. In my experiences only the scaling of the render is the most complex part of the code. which is already done.
The only ways I see this not being possible is that the computer attempting to do it has a resolution <640x480 or this program is using an api that will crash if this is attempted.
Fullscreen: Fills the screen, no trace of the OS's GUI, can't move the render or tell it is in a window (Depending on the OS)
I agree with Simon. I attempted running PT full-screen once and greatly disliked how it looked. What were once single pixels were suddenly squares 6x6 or larger!