Its the SP2010 code and the example shows how to maximize dialog window. You may refer it for SP2013 with the latest changes.

> $z function switches the state of the dialog from "normal" to
> "maximized". The $S_0 variable is set to true when the dialog is
> already maximized. Checking if this is the case avoids from switching
> the state of the dialog at each page refresh.

    function maximizeWindow() {
        var currentDialog = SP.UI.ModalDialog.get_childDialog();
                 
        if(currentDialog != null) {
            if(!currentDialog.$S_0) {
                currentDialog.$z();
            }
        }
    }
             
    ExecuteOrDelayUntilScriptLoaded(maximizeWindow, 'sp.ui.dialog.js');

Source : http://www.wawawum.com/blog/post/SharePoint-2010-How-to-maximize-a-Modal-Dialog-in-JavaScript.aspx