The crura anchor on the ischiopubic rami, far back toward the ischial tuberosities. On me, the inner penis is over five inches, as measured from the points of crura attachment to the junction of the penis shaft and the mons pubis (exit point).
As for suspensory ligaments, there are actually two: the fundiform and the triangular. The fundiform is an extension of the abdominal fascia, which passes over the pubic symphysis, splits into two lamellae that pass around the penile shaft and fuse ventrally with the penoscrotal septum. This ligament acts like a sling, suspending the penis with only moderate attachment to the penile fascia, and is the reason why aggressive downward and BTC stretching can make the abdomen sore. The triangular ligament attaches to the anterior face of the pubic symphysis, descends directly, and fuses with the lateral aspects of the penile fascia. This is the ligament that provides the most support for the penile shaft, and is, most likely, the lig most worked by stretching and hanging.
One other thing to consider is that the suspensory ligaments (and the flava ligaments of the spine) are predominantly elastin, not collagen, unlike all other ligaments in the body, even the crura. This suggests that a different strategy is needed for stretching the suspensory ligaments versus the crura and the tunicae. A widely held idea is that gains occur when very small tears are generated in the collagen fibers of ligaments and fascia, which are then ‘filled in’ during repair, making a longer fiber in the process. This process is confounded when the bulk of the ligament is elastin, since elastin is much more extensible than collagen. In other words, think of collagen as a rope and elastin as a bungee cord. In order to get a really good suspensory ligament stretch, a guy would have to completely extend, or fatigue, the elastin before any real stress can be brought to bear on the collagen. One would also have to keep the elastin in a maximally extended state after collagen tearing to achieve optimal collagen fiber lengthening. The technique suggested by this thinking is a moderate stretch, followed by a short-term, hard stretch, followed by a long-term, moderate stretch. This would optimally be done by hanging, but can also be achieved by manual stretching, as I do. The problem with manual stretching is that the follow-up, moderate stretch is tedious as hell unless one has a traction device or a weight that allows mobility.