You cannot use var in a field, only on local variables.
But even this won't work:
Site master = Master as Site;
Because you cannot use this in a field and Master as Site is the same as this.Master as Site. So just initialize the field from Page_Init when the page is fully initialized and you can use this:
Site master = null;
protected void Page_Init(object sender, EventArgs e)
{
master = this.Master as Site;
}