Gathering Knowledge - Recommended Websites
On December 21, 2008, this website went officially online and is continuously expanding.
It contains a lot of useful information on various subjects, including Tables for Size comparison in DIN.
Founded by a gentlemen called Werner Sölken who is active in construction over 40 years in Oil and Gas and chemical industry. He specialised himself in piping and static equipment.
He is also one of the few people who uses Twitter for good instead of the usual hate speech, harassment and offensive content.
I highly recommend following him!