Much like a fighter pilot must pass a heavy set of stringent physical requirements to even get the chance to fly a billion-dollar war machine, when it comes to being a Rockette, good genes can go a long way. Besides the obvious proficiency in tap, jazz, ballet and modern dance, and a knack for unison high kicks, Radio City Music Hall's signature dance troupe mandates that each of its leggy performers be between 5 feet 6 inches and 5 feet 10A« inches— no more, no less. While this lack of equal opportunity still has its fair share of critics (especially for would-be audition candidates), such soft discrimination has ultimately demonstrated to be quite essential to the production's consistently beautiful all-as-one chorus line. Taking the popular “Christmas Spectacular” on the road for the company's largest holiday tour, the Rockettes are scheduled to perform their lavishly decorated revue with four shows in two days beginning Wednesday, Dec.23, at the Richmond Coliseum. $50-$70.