I'm trying to make a routine that gets the width and height of a device in
inches:

Here's what I came up with:

public void get_display_size() {

DisplayMetrics metrics = new DisplayMetrics();
getWindowManager().getDefaultDisplay().getMetrics(metrics);
float width_in = metrics.widthPixels / metrics.xdpi;
float height_in = metrics.heightPixels / metrics.ydpi;

Or is there a better way?

Assuming you meant the device's screen sizes (you wrote "of a device", not
the screen size) then the sizes are correct for most of the time.

When the app is running on a device that has no physical buttons or running
on an ICS+ device while being targeted lower (thus causing the menu bar to
show) the screen size is reported sans that bar.

As an example, a galaxy nexus while being 720x1280  (4.65 inches diagonal)
is reported as 720x1184 (4.36 inches diagonal).