I am planning to trip to Croatia in the fall and considering taking a ferry from Split to Dubrovnik. I have tried to find information from the ferry websites but there is little to nothing regarding luggage. My thought is that it would be the size and quantity similar to that allowed by an airline since that is what many tourists have entered the country with. Or is it something different?
If you just have a couple bags, as you described--what would be manageable on a plane--I would not worry about it.
Jadrolinija has a reference to an "extra baggage fee" of 50 kuna, but I could also find no descriptions of what that would entail.
I did see a reference on one site that on catamarans, there is a 20 kg allowance: https://www.ferrycroatia.com/en/jadrolinija/
On my multiple ferry/catamaran rides, I never saw anyone checking (but I had backpacks then).
If you can carry it, you can take it aboard. About like a train more so than an airline.
No one checked our luggage on any ferry we took. Like a train they have cubbies where you can put your luggage when you walk on.