What size cement block is acceptable?

I am currently involved in a tier 3 data center design where a portion of an old factory will be converted. The wall that will be the used to isolate the data center space i.e man trap and core network center was originally designed with steel stud and mesh with 5/8 drywall however we feel cement block would substantially increase the perception of security to customers. What size block would be acceptable to maintain tier3 standards, 6″, 8″ or 10″?
Thanks