Hi Andy,
Section 9.1 of the Hardware Design Guide for KeyStone Devices discusses the layout guidelines for the BGA. If refers to SPRU811 Flip Chip Ball Grid Array Package Reference Guide which contains land and solder mask information in section 3.2.1. Table 2 in that section specifies the PCB copper land for parts with a ball pitch of 0.8mm as 0.45mm or approximately 18mil. Section 3.2.1 contains details on the reason that this is the best pad size to use.
Regards, Bill