You can select the desired paper size in File>Document Setup>Paper. A2 is list in the drop down near the top of the list. If you're modifying an existing template that shows the grid lines, the grid lines will still be displayed with the same spacing. You can go to the Grids page and make changes to the grid spacing, color and whether or not the grid will be printed.
While you're in the Paper window, you can adjust margins and other details, too.