If you’re creating an image, decoding an image, or resizing an image to fit the large notification image area, you can get the correct pixel dimensions like so:

Resources resources = context.getResources();
int width  = resources.getDimensionPixelSize(android.R.dimen.notification_large_icon_width);
int height = resources.getDimensionPixelSize(android.R.dimen.notification_large_icon_height);