Well I must admit since I know that ships will only get larger I am naming the NCNs ships based largely on the idea that eventually the name will fit. I could call the Terriers/Guardians/Londons frigates, the Wounded Knee/Gargoyle/Lake class destroyers and the Tribals either cruisers or battleships.
But because that would mean when the next size of ships comes out I would be re-classifying them I stuck more with a naming convention based on size but also role. The frigates (Escort, Heavy Frigate, Corvette) classes are the smallest warship and they each are mission specialized. Destroyers are the first multi-role ship but one that is not capable of independent operation. Cruisers are multi-role (in general) ships capable of indepentent operation. Battle cruiser doesn't make sense to me and won't be used. Battleships are large warships intended for decisive combat situations....since there is nothing more decisive than a Battleship on Battleship engagement.
"Independent Operation" here means: does not require a jump ship.
Other than that the NCN differentiates inside the class by role. But due to inter-building being fairly key to the Navy design all ships of a certain "class" will likely be the same tonnage. Also every ship carries the sensors needed to do its job (search, missile, IR, EM) to prevent mission kills by single ship losses. But designation wise the Gargoyles are FGE(H) or heavy escort frigates while the Wounded Knee's are just FGH. The Edinburoughs are classified as Supply Ships which makes them "SS." The game "War in the Pacific Admiral Edition" has a listing of ship classifications abreviations and it is quite extensive, and to effectively play the game you have to get familier with them...
I think how you define your ship will in many ways be determined by what the state your game is in. If you are like me in the process of building your navy and have limited hull sizes available then it is likely you are going to name your ships by hull size using names that reflect smaller ships. If on the other hand you do a regular start where you can begin with substantial ships then you can adopt the more modern convention of name by role since it is likely that further expansion will be on a more modest scale to a conventional start.