The android way to get screen width and height is DisplayMetrics().
by default, if you don't write size(…,…) in setup(), processing get the screen size and make your sketch fit to it (ouch ! sorry for my poor english).
Positions/scale of objects should always be set according to relative value from width and height.

Now if you want opengl render, you have to set the size of the sketch.
A little search into processing sources gives: (I search for DisplayMetrics()).

1.     DisplayMetrics dm = new DisplayMetrics();
2.     getWindowManager().getDefaultDisplay().getMetrics(dm);
3.     displayWidth = dm.widthPixels;
4.     displayHeight = dm.heightPixels;

It's very instructive and useful to look into sources. Don't hesitate to do so.