From a USSA Publication dated September 26, 2003 . (Sorry. I have no URL link to it on the web.... however, this text is a portion of a document distributed to officials as clarification)
Due to the nature of USSA competitions, considering the scope of abilities, ages and physical maturity, the USSA does not specify competition rules regarding ski length and radius for USSA events.
USSA Competition Regulation 18.104.22.168 "equipment must conform to USSA regulations", this document restates the USSA regulations regarding ski length, ski radius, ski boot thickness and ski/binding/plate thickness.
The responsibility for skiing, training and racing on appropriate equipment is up to the racer.
There are restrictions for side cut and length of skis for all levels of FIS racing (including International Children's and International Masters). Athletes competing in FIS events must compete on equipment that is in compliance with FIS rules.
USSA does not currently plan to institute rules regarding ski length and radius for future seasons. However, this matter continues to be reviewed, and if the need arises USSA will institute such rules.