Another way to get the numbers (close enough) is to set the terrain's position to lower left at 0,0,0 and then zoom in to the top right corner of the terrain's bounding box. Position the mouse over the corner and read the coordinates at the bottom of the preview window. Remember it's in metres, so if the coordinates are in km you need to multiply by 1000. X is width, Z is height.
Set image projection to Plan Y and position with the same settings as the terrain...