Firstly, though I've been working on some B5 stuff earlier this year, this write up is not mine. Everything I do is under the single "Babylon 5" section in the D6 Starships area of the site, while the previous entries were done by someone else (including this one) and all were placed under various Babylon 5 titled sections (the categorization is simply a difference of preference between me and Freddy, and just depends on which of us posts what on here).
Secondly, I am not so sure about the size of these ships. I have yet to read through the Agents of Gaming RPG books. I know there have been RPG related books in the past, but did not know if they were all related or not. The RPG books seemed "iffy" to me for various reasons (see a previous conversation in the comments section of the Warlock Destroyer between me and DemonApostle for more info), mostly due to lack of accuracy on finfo and statistics. However, since you say JMS invests so highly in AoG's books, I guess I'll have to give them a try. But, all in all, I have yet to see any official statement on the size of these ships. If I were to do them up, I think I'd do both a "young" and "primordial" version, with notes to alert GMs that certain stats could be altered to fill the gap in between these two as they age.
Thirdly, from a personal standpoint, when doing up stats, I will hunt down whatever info I can find, but I also go by whatever info I can glean from the original source. In this case, for the stats I have written and posted here, I was watching through the entirety of the Babylon 5 franchise at the time. It got me on a kick and I wanted to do some stuff for it here. Watching it all helped me when doing up stats, getting balance issues right, but also doing this in comparison to Star Wars ships. All in all, from my own personal perspective, i was happy with the outcome.
Fourthly. You say JMS backed AoG's RPG books as canon? I'm interested in this. Can you provide a link to something online stating this, or tell me where to find this information? I'd feel much more confident in using AoG as a source of conversion material if I knew this for myself.