Ships of larger size are not in the least more fragile, unless maybe when you fight serious amounts of mesons.(but a smart commander knows how to avoid that) Otherwise they turn out to improve even on the size/durability ratio, as for example shields begin really to shine the larger a single unit is (exploiting shield regeneration), and also armor does weight less per layer due to improved surface-to-volume ratio of bigger ships, so you can easily have 10+ layers and only have it weight less than 5% still. (not to mention that enemy damage scatters a lot more over larger hulls, so critical hits {~hitting the same spot luckily and achieve penetration} become immensely unlikely statistically on those giants)
Engines and costs are not a problem too. You don't build these ships right at the start of course, but I have found myself being able to put 100kt designs out as early as 40-50 years into the game easily with home system resources. ...And later there comes an age in the game where such abundance of resources exists, that you can just afford huge ships without bother (or "abuse" of sm mode...
- I myself never cheated so far, except maybe for tiny design corrections when something was bothersome).
At the bottom of
this post I have a 300kt cruiser that actually costs so little that I could easily afford an armada of 20 of them if I wanted to. (..only reason I don't is because I find it bothersome to micromanage too many carrier role ships, and because I need the hangar to store them too)
Fuel is not an issue either as soon as you have at least one valuable gas giant in your supply chain. (making fuel on ground is ridiculously ineffective compared to this) I ended up storing billions of liters extra fuel in any of my games despite only using large ships. They are actually more fuel effective even, because you use more efficient engines on those, and give the hunting to faster fighters/bomber.
Other occasions where I set out to build huge ships:
http://aurora2.pentarch.org/index.php/topic,7297.msg75070.html#msg75070http://aurora2.pentarch.org/index.php/topic,6721.msg68767.html#msg68767http://aurora2.pentarch.org/index.php/topic,6738.msg68949.html#msg68949...The fortresses scratch on the limit of what is possible and affordable in Aurora, as I had to strip mine valuable gallicite planets for some years to make those possible. Setting 3mt battleships into space costs near to nothing when you are 100 years into a standard game however.
Don't judge it before you have tried it. It works well.