If you're referring to Processing's IDE, you can modify many of its defaults
@ "preferences.txt" inside Processing's install folder.

Now, if you wanna change 100x100 default screen size of a running program, you may try this out:

final static int GW = 800, GH = 600;

@Override int sketchWidth() {
  return GW;
}

@Override int sketchHeight() {
  return GH;
}

Thanks, GoToLoop - changing the preferences is exactly what I needed.

That is good to know. That has been bugging me...