how to get android screen size programmatically, once and for all?

Question

Display display = getWindowManager().getDefaultDisplay();
Point size = new Point();
display.getSize(size);
int width = size.x;
int height = size.y;

Now you can measure your screen size in pixel which is a better measurement unit than centimeter because all the buttons ,textviews etc.. are measured in this unit. That what I use normally

By using the DisplayMetrics you can get height and width of the screen of any device. Note that width and height are sensitive to rotation. here is the code.

DisplayMetrics metrics; 
int width = 0, height = 0;

In your onCreate Method

 metrics = new DisplayMetrics();        
 getWindowManager().getDefaultDisplay().getMetrics(metrics);

 height = Math.min(metrics.widthPixels, metrics.heightPixels); //height
 width = Math.max(metrics.widthPixels, metrics.heightPixels); //width

Try this.

In your #6, you note that the DecorView seems to provide what you want, but you don't think it is realiable. I believe that is as close as you can come. According to the documentation for Window.getDecorView:

Retrieve the top-level window decor view (containing the standard window frame/decorations and the client's content inside of that), which can be added as a window to the window manager.

The status bar is part of the window decoration mentioned there. Software keyboards and other overlays will receive their own window, so they shouldn't interfere with the relevant values. It is correct that isn't precisely the display metrics, but if your application is full screen it should always be the full screen size filtered through the compatibility translator. Other applications won't have access to your application's Window, so there is no need to worry about some other app changing the attributes while you aren't looking.

Hope that helps.

I should put this as a comment but I don't have the rep for that.
This may not be a totally correct answer but I got my dimensions when I switched the height and width in the DisplayMetrics method.

    DisplayMetrics metrics = new DisplayMetrics();
    getWindowManager().getDefaultDisplay().getMetrics(metrics);   
    width = metrics.heightPixels;
    height = metrics.widthPixels;

as I said this may not be correct but I don't know why but it worked for me.

If you are using api level 17 or higher check out getRealMetrics and getRealSize in Display

For api level 14,15 & 16 look here

I've used Pythagoras theorem to find the diagonal size of Android phone/tablet screen, same principal can be applied to iPhone or Blackberry screen.

DisplayMetrics met = new DisplayMetrics();                
this.getWindowManager().getDefaultDisplay().getMetrics(met);// get display metrics object
String strSize = 
new DecimalFormat("##.##").format(Math.sqrt(((met.widthPixels / met.xdpi) *
(met.widthPixels / met.xdpi)) +
((met.heightPixels / met.ydpi) * (met.heightPixels / met.ydpi))));
// using Dots per inches with width and height

Pythagoras must have been a genios, he knew smart phone programming so many years ago :p