//show the dialog first
AlertDialog dialog = new AlertDialog.Builder(this)
        .setTitle("Test Dialog")
        .setMessage("This should expand to the full width")
        .show();
//Grab the window of the dialog, and change the width
WindowManager.LayoutParams lp = new WindowManager.LayoutParams();
Window window = dialog.getWindow();
lp.copyFrom(window.getAttributes());
//This makes the dialog take up the full width
lp.width = WindowManager.LayoutParams.MATCH_PARENT;
lp.height = WindowManager.LayoutParams.WRAP_CONTENT;
window.setAttributes(lp);

import android.support.v7.app.AlertDialog;

import android.app.AlertDialog;