I am a fan of big as they can be Carriers. Heavy Shields, a bit of Armor, DC/Repair, Countermeasures, HyperDeny, and then all the Bays that can fit.
For instance in a recent game ship size at 1100 base, my Carrier was 1649sz with 22 Bays.
At that point I was running 6 main Fleets, each had 10x Capitals, 3x Carriers, and one attached Troop Transport when being used Offensively.
As well as several defense fleets that were 4x Destroyers or Cruisers, depending what era they were build in.
Even running all these ships and Bases I still had 5.5MCr on hand, and a +250K cash flow even after several Financial Collapses the last time I remember looking..