Please select from Menu file - Save As... and choose Big Tiff Image (*.tif) file type.
Units - You may choose Pixels, Inches or Centimeters as a mosaic size units.
Resolution - You need to set this value to allow the program to convert pixels units into inches or
Image Width, Mosaic Height - You have to set one of these values, the other will be calculated
Save - Press this button to export/save file.