Here's a slightly modified version of Naz's example. This version allows a full browser size background map without the need to disable the horizontal scroll bar. This means that your page itself will work normally.
It achieves a full background map by using 100% width and 100% height (with a min-width and min-height the size of the page) for the Div containing the iFrame, and by moving it into the main body above the main WebPlus page Div. The initial disadvantage of doing it this way was that the user could click only on those parts of the map that were outside the confines of the main WebPlus page, however that problem was easily overcome simply by adding a very small bit of code onto the end of the WebPlus page div to set its height to zero pixels. This works fine, with all the page content remaining in exactly its correct position and it allows user clicks on the map all over the whole area (apart from when it is over a specific page element of course).