For most people to answer to this is "as big as possible". Charlie Wing's book the "Boatowners Illustrated handbook of Wiring" has a very good explanation of how to calculate the capacity you need. The calculations start by working out what your daily load will be both at anchor and while underway.
You need to remember that you cannot use all of the capacity your batteries have without shortening their life considerably. You should aim to not discharge your batteries past 50% of their capacity. In addition if you are charging batteries from your alternator while underway you will find that the last 20% of the charging cycle is painfully slow. All of this means that the amount you have to work with for continued operation while underway is 30% of the rated capacity of your batteries.
So for example to provide for a consumption of 100 Amp Hours per day and charge once per day you would need to have a battery bank rated at 333 Amp Hours.
I enclose a sample table below for you to complete in order to calculate what your actual power consumption is.. I have filled in a couple of lines with a hypothetical example. Every boat will be different of course and you will have to make a best guess at the answers. You should do a separate table for when underway. Often at anchor the loads will be higher as there are more lights left on for longer times and they are not offset enough by lower navigational equipment loads.
|Watts||Watts / 12 = Amps||Time|| Amp |
|Fans and Blowers|
|Galley or head|
| Total Amp Hours|
per day at anchor.