Vessel Particulars

What is the best reference for finding general particulars on a vessel (beam, lenght, draft, etc)?

The Pilot Card.

If youre not onboard, that information is usualy posted with varying degrees of accuracy via marine traffic.

If You are not a seafarer try AIS tracking websites for general particulars, then get IMO number and then log in to Gisis and Equasis and You will get more then general particulars.