More than 55,500 students are enrolled here at Arizona State University, many thronging its paths on bicycles, scooters and longboards. By most reckoning, this sun-drenched campus is the nation's largest. Counting three other ASU campuses in metropolitan Phoenix, the mega-university's enrollment is 68,000, up 24 percent in seven years. University President Michael M. Crow has said the total could eventually approach a staggering 100,000.