ANSI/HFES 100, titled Human Factors Engineering of Computer Workstations, is the ANSI standard for computer workstation design specifications. It is the ANSI counterpart to ISO 9241 and ISO/IEC 9995. It includes design guidelines for various input devices including keyboards. The current version is ANSI/HFES 100-2007.
Section 6.2.2 discusses keyboard specifications.