The Morshin is definitly a carrier in "Babylon 5 Wars: Ships of the Fleet", but also just a "heavy combat vessel", in about the same weight and size class as the EA Artemis. It can carry 48 fighters, which is REALLY a lot, the EA Avenger Heavy Carrier can fit the same, but is one size class larger and, by the ramming factor in the tabletop rules, must be approximately twice as massive. So, yes, it MUST be small.
From the fluff text: "Among the small of the Minbari ships, this was designated solely as a fighter delivery system, [...]"