Very good point about classes being a largely political construct imposed on warships, as well as the logistical advantages of a single-design navy.
However, it's worth remembering that battleships were supplanted by carriers because of the carrier's ability to attack from beyond a battleship's reach, and deliver even bigger and more accurate salvoes in the form of heavy bombs.
Actually that's not quite the point. It's more the understanding that just being able to fight is almost worthless. It's not about being able to fight one another, it's about achieving the mission objectives. Commerce raiding versus commerce protection was the key aspect of the Cold War, and battleships had no place there. If on the other hand say naval invasions were a critical aspect, battleships would have stayed around a lot longer, since it's only relatively recently that carriers could operate at night or during bad weather. And enemy ships getting into the landing fleet or a supply convey unloading onto a beach during night would be an utter disaster.
In space, small craft are not categorically faster than large craft, since people pose a bigger limit on acceleration than the non-existant drag. Missiles have no easily available cover to hide their approach, and it's basically impossible to achieve stealth against the background of deep space. This means that point defences will have plenty of time to intercept, and the high velocities involved means that "grapeshot" would murder incoming projectiles.
Add in the ridiculously effective Battletech ablative armor, and warships - whether true-and-pure Warships, or combat-dedicated "dropships" - will slaughter small craft by the score with their direct energy weapons.
Actually it all comes down to in fiction technology. How effective is inertia dampening for vessels of different sizes? It's easy to say that small vessels like fighters have ten times the acceleration. And if cloaking devices exist, missiles could be hidden well enough. Or they could have energy shields powerful enough that fast firing fast tracking weapons that'd be ideal for point defense are useless, and with good enough ECM missiles will get through. With the right technological justifications, everything can make sense.
And in a paradigm where size matters, ships naturally develop into multiple strata - first-rate, second-rate, third-rate, etc - as the great powers struggle to balance the need for super-ships to fly the flag and guard key worlds, against the need for large enough numbers to not only guard their holdings but patrol their space.
Not necessarily. It depends a lot on technology, how stable it is, and politics. So technically anything is possible, but certain limits exist. But in the context of this story, super ships make no sense. The Alterans were unchallenged and had ships that could get anywhere in the galaxy in minutes. Do you need anything but a limited number of the best possible ships in those conditions?
Even when challenged, would developing a cheaper ship type to produce in larger numbers been cost effective for them? For a quasi immortal race, expendable ships would be strange.
Of course when you go to other universes and remove the incredible strategic speed, things change. Even introducing massive fortifications to crack might change it - then again the Alterans might be exclusively using maximum (useful)size ships; as in any larger and sub-light engine performance drops massively or something of that nature.
No. Battleships were supplanted because firepower surpassed armour. Ships could not be adequately armoured against firepower carried on much smaller vessels and vehicles. You no longer needed battleship-scale guns to shoot down battleships.
In space larger craft generally means higher endurance, longer missions. And if you DON'T ass-pull some ridiculous special super-armour to justify them being "better" than smaller craft they STILL have a purpose.
Firepower played a role, mostly in how effective subs were getting in killing ships, but the more important part was the Cold War challenges were all about lines of communication and keeping them open/cutting them. There was literally nothing for battleships to shoot at.
Space battleships could be justified, as I said, if shields or armor thickness could be piled on high enough to make cruiser weaponry meaningless. With armor that's unlikely to be possible, since mission killing a ship doesn't even require penetrating the armor, but with shields? Shields operate pretty much at the author's discretion.
But large does not necessarily mean high endurance or longer missions. For one fleet tenders/civilian transport ships can bring along supplies, rendering individual ship size meaningless. For another, given a certain tech level, like in this story, energy becomes the only resource you need to carry along; with potentia batteries decades of supplies at max consumption would take up less space than a crew cabin. Of course speeds are so high that supplies carried along become nearly meaningless; home base is literally never more than a few minutes away.
For different tech bases that changes, or course, but there are also counter examples. Take Star Trek; the main limitation to ship size in universe is the Warp Drive. The larger the ship, the more complicated the warp field gets; energy consumption explodes while speeds drop. The long range vessels of the Federation are fairly small sized vessels like the Intrepid; perhaps an eighth of the volume of an older Galaxy class vessel, they combine high speed with endurance that at least matches the Galaxy class.
Then of course there's the question of what kind of mission you'd send a navy vessel that it needs extra supplies. Even six month missions put an extreme stress on families. If you try for longer missions, there'll be a huge drop in the quality and quantity of volunteers. Yes, blue navy ships tend to be bigger than brown navy ships, but you don't see any navy mix blue and brown navy ships; they are either one or the other.
The last two points are important to always keep in mind; what kind of missions do you need ships for and what kind of deployment schedule is the crew willing to tolerate. An emergency mission that needs more endurance can always take a transport ship along to extend their range without requiring a dedicated new ship class for rare and unusual missions. And if long missions are very common? It makes more sense to have all ships capable of performing them, so that you can rotate ships and crews and don't have one sub-section of the sailors under far more stress than the rest; and it's far more flexible when ships get damaged and things need to be shifted around on the fly.