There is absolutely nothing wrong with calling a 30kt ship for a destroyer in aurora as ship sizes are completely relative to the type of economy and what you can support and research. Size are generally dictated by the type of components you put into the ships such as Jump Drives, Engines, shields, cloaking devices etc... components that you can scale with size to make big ships even more powerful.
If you have the economy/research to support large ships I see no reason why you should not build them. Large ships have allot of advantages but small ships also have advantages.
I would say that a healthy mix of different ship sizes for different reasons are quite useful from a strategical perspective.
In my games ship sizes simply grow with time as the economy and research progress, this seems like a very natural progression.
Small ships have a big advantage that you can produce them faster per tonnage and they are stealthier. Large ship have the benefit of being allot more efficient per tonnage and much harder to destroy.
Investment in large ships is a long term commitment. Small ships are better because you can ramp up very high production of new high quality ships very fast. It is very easy to construct one or two 1000t yards where you can mass-produce 25-50 FAC per year. But you will most likely see these yards dormant 95% of the time as the only time yo use at full capacity would be during a war.
On the other hand... your two 50.000 ton yards capable of producing three cruisers creates perhaps three ships every four to five years or so... but these ships are extremely powerful for its size and are ships you will nurture and take care of for many decades.. perhaps as much as a hundred years.
Whatever you call a ship should in my opinion be based on their role and not have much to do with size in and of itself... at least that is how I do it. Now... ships do tend to end up in size similarities with reality, but that is because the roles are similar to how those ships were used in reality so the tend to fall into the same type of spectrum.
So... on the bottom scale of capital ships I usually have the frigate. This would generally be a ship that are in some way self sufficient and tend to operate alone. They tend to be highly specialised ships in some way such as reconnaissance, scouting, patrol or similar duties.
The next would be the typical destroyer and these could vary in size quite allot even within the class... mostly because they will have been developed over many decades in many variants. Older destroyers tend to become destroyer escorts with the task to protect commercial shipping in all forms while more modern destroyers are the main protectors of other capital ships or who perform reconnaissance in force missions. These ships need to be enough self sufficient to operate in small squadrons alone.
The next class is the cruiser who is a large capital ships tasked with long range patrol and high strike capabilities without supporting elements other than destroyers. They will tend to be a fair bit larger than destroyers as they are meant to actually engage enemy by any means necessary so will need to perform will in all fields.
By the time I get to more specialised larger capital ships i generally have neither carrier or battleships but rather battle carriers... a battle carrier is a monstrosity of a ship designed only to for war and to do it very well with a combination of powerful beam weapons and strike crafts. They should be able to perform in any capacity on the field. A ship this size can't be lost because they are caught in a bad situation.
The actual size of any of the above ships would depend entirely on the circumstances... destroyers tend to be about 10-20kt in most of my games while cruisers are 30-50kt and battle carriers at whatever size I can build them.
In most early games I would not have much of any capital ships at all since I would not have the technology to really support them so I would have to make due with a mix of frigates and destroyer type ships. I would also have many other different ship types such as light carrier, escort carriers, assault carriers, jump tenders, command ships, mine layers and more...