the donor has to be the same blood type or blood type O, in good health and be about the same size without any liver disease or appreciable medical problems.
Sorry the link I posted was incorrect, the correct link is:
http://www.transplant.bc.ca/pre_livingliver.htm then select
Information guide for potential live liver donors
For a guide to the Living Donor process see: http://www.transplant.bc.ca/living%20liver%20donor%20guide_electronic.pdf. It is a good overview of the entire process and explains the compatibility issues involved.