I don’t believe there is any solid canon on the exact size of the Iapetan Titans, except that the current, “Promethean” Titans are much less resource-intensive versions. Now, when the Erebus is a little over 14km long, it can be assumed the original Titans would be at least twice the size, possibly more.
Well, we could maybe take the Triglavian World Ark to represent the presumed size, as it is apparently designated to be the largest known vessel to officially exist, with it having a “wingspan” of 55 kilometres. Of course we don’t know if the Iapetan titans are an officially recognised shipclass, but, if so, then we know the upper limit on the dimensions of them, which would be around 55km long, wide or tall.
The idea that they are moon sized probably stems from the above linked article, and/or player interpretations of it, where it is mentioned building two of the Amarrian Emperor Ships “had stripped an entire mineral rich world bare”.
And from there we get to the number issue, all Empires are assumed to own at least a handful of Iapetan Titans and countless of the smaller, Promethean ones. I can’t provide a source, but I recall a CCP story dev saying at some point that the Gallente Federation considered deploying an entire Soltueur wing to dislodge the Caldari Titan from orbit of Caldari Prime if all else failed.
If you mean with “only two exist in the game canonically” that there were only two models ever made for them, that is I think correct. And of the two, we’ve ever only seen the Gallente one once in-game, and that was in, like, 2003 or 2004. “Leviathan” (the Minmatar one) I thnk also had a model datamined, but never actually seen. Imud Hubrau and (presumably) the Caldari one that doesn’t have a name at least known to me, only have very old artwork from before the game even released.