Every ArcMap document has a defined page size and page units for its layout. By default, the layout size is set to 8.5 x 11 inches. A GET_LAYOUT request to an ArcMap service requires layout page size (specified as ENVELOPE) for proper generation of the layout.
This information can be obtained using the GET_SERVICE_INFO request to the ArcMap service.
The page size and page units for a layout is provided within the <LAYOUTINFO> .. </LAYOUTINFO> tags in the response for GET_SERVICE_INFO request.
The attribute 'pageunits' defines the units for layout page. The layout page size is returned as an ENVELOPE with the LAYOUTINFO.
Here is a sample response which contains layout page information for a 11 x 8.5 inch layout.
<?xml version="1.0" encoding="UTF-8"?>
<ENVELOPE minx="0" miny="0" maxx="11" maxy="8.5" />