While no specific form factor standard is specified in JAS it is highly recommended that programs make use of a standards-based mechanical design such as VPX defined by VITA.
Although VPX is preferred, other standards may be considered for use as well. The consideration of alternate standards should carefully consider a number of factors as detailed in this Mechanical Specification.
"VPX: Base Specification". ANSI/VITA 46.0, 2007 (R2013), VITA Technologies (VITA), 2013.