I found the study. Some interesting quotes:
"The engineering analysis predicted that a minimal tension force of approximately 450 g during stretching of the penis is required to reach the potential erection length. The stretching forces exerted by the urologist in the clinical setting were experimentally shown to be significantly (P<0.01) less than this value."
"Using these diagrams, the stretching force that should be used to predict the length of the penis during erection can be estimated, assuming normal intracavernosal pressure and tunical elasticity. It was thus clearly demonstrated that in order to obtain the most accurate prediction of penile length during erection, ie to approach the value Le=Ls 1, a stretching force of around 450 g (~4.5 N) should be exerted."
Also note Table 1:Average penile dimensions (in cm) during flaccid, erect and stretched states (8.3 vs 13.6 vs 12.5 cm respectively).
Also extensibility seems to decrease with aging:
See figure 1. "However, as in the penis, urethral extensibility decreases with aging. In older patients the urethra is less extensible than in young patients, which should be considered at surgery."
http://www.urog enitalresearch. … 2042_5_2002.pdf