I knitted in 2 threads of Plötulopi and felted in the washing machine at 40 ºC (104 ºF) with a little bit of detergent and a pair of jeans. They shrank 20 %. From a lenght of about 40 cm to a lenght of 32 cm.
Here is a picture of the underpants before felting:
And here is picture after felting:
Now I just hope they fits my boyfriend. We will see tonight. I checked with a pair of his normal underpants. They should be fine.