There are a lot of factors that go into determining the appropriate sized battery bank for a boat, and no hard and fast criteria. However, there are some general guidelines that can help in setting up a bank of batteries that will provide enough power for the DC systems on board without sinking the boat or your finances.
The first consideration, for sizing, is the type of boat and how it is used. A cruising boat has quite different needs than a fishing boat or a work boat. Even amongst the cruising boat crowd there are those that spend very little time away from a dock, and those who spend long periods of time at anchor. Batteries are also heavy and occupy physical space a well, so weight and space are considerations. Batteries (especially LiIonPO4) are also quite expensive, which is a big factor. So all that being equal, the bank can be minimally sized by considering loads and the capacity required to keep critical systems running for an acceptable time frame.
If the electrical system is well designed then it should be possible to turn off the main battery switch and still have power to essential services like bilge pumps and alarms. A bilge pump will require approximately 5A to 10A to run; true peace of mind would be capacity sufficient to operate at least one pump for 24 hours. On a smaller boat, that is about a 250Ah bank and in the larger boat about 500Ah. Other essential equipment might include navigation lights, emergency radio and alarms.
Capacity beyond this minimum will vary greatly depending on the other factors, for some boaters the ability to run a cappuccino machine or a drink blender maybe considered essential equipment.