Honestly I really quite appreciate this change about the hull types. Although other players might've enjoyed the "freedoms" of DW:U pertaining to ship types, I always found it irritating that escorts, frigates, and such could theoretically be larger than carriers or capital ships.
With the new system, it'll be much easier to gauge the power of specific ship types because you'll understand that a frigate, for instance, could never have more than "X" amount of hull size. This, in my opinion, is an improvement. It works to further define what a frigate actually is. This change hardly hampers any creativity of the player. It works to specialize each ship type within more realistic boundaries, while simultaneously giving the player another layer of preference to consider (hull types) when designing ships.
Edit: Not to mention that when ship types have specializations within a set of boundaries, it allows the developers to implement systems for specific ship types in ways that would've otherwise been impossible.
I know this is probably going Out of topic, but why do you think a Frigate needs a size constrain? why would you think that is realistic? I mean going irl, ship size is a matter of the mission, a ship is as big as it needs to be and as cheap as you can make it (specially if you need them in numbers).
as for Frigates bigger than BBs, I can see some scenarios where you can have Battleships that might be, in fact, smaller than frigates, say a slow moving "coastal" battleship, meant as an alternative to a defense station that can be more or less quickly moved into place to provide defense to a colony till you can get up some proper facilities on site (few fuel tanks, few engines) not sure if it would work, but as a design idea it is interesting.
Honestly I dislike inherent bonus to a hull class, better have those be done with components than an artificial modifier attached to the hull, since that design is a pain to balance (look at stellaris and the corvette issue for instance)