Foto 3D will print mats up to the size of the page in the printer. If you have access to an A3 printer for example, you can print a larger mat. Foto 3D will also print larger mats across a number of pages which can then be glued together. Make sure the resulting mat is flat and accurately produced.
You can print the mat from within Foto 3D, or you can find the Calibration Mat.pdf file (located in the application folder) and print from that.