There are endless debates about ship size all over every GW related site because there was never any effort to clearly set out precise figures.
A consensus among fans was building towards 750 metres for cobra destroyers, 1.5 km for a sword class frigate, about 3 km for a lunar class cruiser and about 5km for a battleship. This consensus was based upon some comments from people like Andy Chambers and Richard Williams, both published GW writers.
However, recent designer diaries for the Rogue Trader game hint at a alightly larger scale: about 5 km for a lunar class cruiser and scaling from there. (so yes, about 1.5 km for a cobra destroyer.)
I guess that technically Rogue Trader doesn't need to address this issue, because most of the Player Characters will be using smaller craft: however, I'd quite like Rogue Trader to have some detail about this stuff, because I love having as much detail as possible about the 40k universe, and the RT game seems a logical place to sort this all out once and for all.