If you're using Trinity make sure you modify the Diplomacy.user.settings file. The regular user.setting file is for original/Entrenchment.

I think it has to be one of the standard sizes in the set in Options --> Video.

Not sure how TVs work, it might just act like a DVD movie that is not the size of the screen adding a black section on the top and bottom or the sides. I have tested different sizes on the monitor with no problem. Fills the monitor but slightly stretches or squishes depending on the size. I have seen different size settings on new TVs as well.