I found on the internet a document created by Will, dated 24APR12, where the Hydran ships were drawn out for measurement. It appears the relative sizes were gathered from 2400 line of minis. My Google-Fu skills fail me on finding the original document (I made a personal copy, but am loathe to post that without permission.)
This image, and the sizes derived therein, have not yet had a personal vetting from any SFB Gurus yet. It's just cobbled together from references found in Section-R and what's been gleaned from the "main" BBS.
I don't have a Fed CA on hand. But I do have a Burke class Fed FF. My source (which is, in this case, Garth and Will measuring minis for Mongoose and Nick) says the FF is ~95 meters wide.
To answer the question before it's asked, the models were created from the reference drawings made for
the 2400 minis. In the case of the Fed ships (of which I only have the FF, ATM) the same technique was used but from the images put out by Vanguard.
(I apologize for the shoddy texturing on the Ranger's port side. That will get fixed when the fighter tubes get fixed.)

(Click for larger image)