Skull size of Homo naledi
From my research, I can deduce that the cranial volume for males is 


  560 cm3 (34 cu in)


and


  465 cm3 (28.4 cu in) for females


Note: These skulls are approximately half the volume of modern human skulls average Homo erectus skulls are 


  900 cm3 (55 cu in)


I don't think you would need the area for 3-D printing, but you can use the volume!