Chastity and orgasm denial discussion

What is the ideal dimension for a chastity tube? 1- Long enough to allow partial or complete erection? 2- Small enough that erection becomes unachievable? 3- Tiny, matching your penis when it shrinks in cold water or ice? My initial tube was size 1, but currently I stick with size 2. My observation is that the body quickly gets used to the notion of having no erections at all, including during sleep and early morning hours. At present, I exclusively use a chastity-tube measuring 70 mm/2.4 in total length (such as the Steelworxx Revenge or Looker), which suits me well. I am considering trying an even smaller one (size 3). It would be great to hear what others have experienced regarding the perfect size?