I suffer from severe ED (diabetic) and for a number of years I used a doubled over 1/2 inch well lubed rubber band around my whole unit (behind the balls) in order to maintain an erection during sex at least twice a week. Although it temporarily increased my girth by over 1/4 inch, I never experienced any perminant increase to my girth.
On the down side, the constriction caused my dick to start to loose sensitivity after ten minutes, it started to go numb at the 20 minute mark, and it had to be removed before 30 minutes in order to avoid damage.
Accordingly, if you have ED, it can be a great help. If you want to encourage penis enlargement, this is not the way to go. However, you may want to look into a stretchy cock ring to generate some engorgement during jelqing.