This is a decision that is entirely up to the user, with some caveats. The general consensus is to size a hard wired charger for 10% of the banks capacity. In this case a 400Ah battery bank would get a 40A charger. Sizing at 10% of capacity or less allows thick plate deep cycle batteries to accept the current deep into the plates for a full charge.
If you size on the low side you must also consider the DC load your boat will use at the dock, when using DC devices. You should consider this load number when trying to get to your 10% sized charger. For instance some power boats & sailboats have small banks under 200Ah. This would suggest a 20A charger. This is well and good except when your dockside DC devices can consume more than 20A, lighting DC refrigeration, computer, TV stereo etc. etc.. In this case you could have a 20A charger sized at 10% of "bank capacity" but still be drawing your bank down, and not charging it, anytime the DC loads exceed what the charger can deliver. As I said, "with some caveats".. Size carefully and don't forget to consider the dockside DC consumption.
The Sterling PCU chargers have no problem charging a large bank and can run at full output for hours & hours on end. The cooling fan on these chargers is a variable output design to let these chargers run quieter. Chargers with single speed fans are most often louder as the fan is either on or off. I have yet to have the fans kick on very often with the Sterling PCU chargers.
Some chargers, usually fan-less units, can not charge large banks without suffering from internal heat build up. This can result is a shortened life for the charger if it is not sized correctly. The more efficient the charger is the less heat it will produce. The Power Factor Corrected Sterling PCU is near 90% efficient which is a boost over non Power Factor Corrected chargers of as much as 40%!
What the heck does that mean? What it means is you'll have a cooler running charger, less noise/fan, a smaller foot print because leas heat needs to be dissipated and you'll use less AC power to charge at the same DC output than a non Power Factor Corrected charger. Even the Sterling PCU 60A model will easily run off a Honda EU2000i generator and leave you with LOTS of left over wattage, about 700W left over, to run other devices while charging your bank at 60A. NOTE: The Honda EU2000i, a popular gas suitcase generator used on small boats, has a constant load rating of just 1600 watts and is not really a "2000 watt" generator for constant loads.
Sizing to the 10% rule, or better yet, the manufacturers suggestion, is a better choice with non Power Factor Corrected chargers as they develop more heat. The Sterling Chargers are highly efficient so being in the 10% of capacity range is not as crucial with these units or other Power Factor Corrected chargers. The "time" you need to charge can be simply based on just that, "time".
If you had a 400Ah bank and wanted it charged from 50% state of charge, to full, over one day, you can get away with a 20A charger. Even if the charger is sized at just 5% of total bank capacity your really only drawing your 400Ah bank down to 50% state of charge. By following the 50% max depth of discharge rule for house banks you would only need to replace 200 amp hours, plus charge inefficiencies. If however you need it charged back to full in 10 hours, well that's just not going to happen with a 20A charger.
Conversely if you power your charger off a generator, when away from the dock as many boaters do, you will want as much charger as your batteries will accept to keep generator run times as short as possible. My one and only real gripe with the Sterling chargers is the largest single charger is 60A. On vessels with large banks or AGM or other types of batteries that have high acceptance rates as 60A charger can limit your recharge times when using a gen set to charge while away from the dock. For larger chargers Victron & Mastervolt make good ones, or simply double up on the Sterling. Using two separate chargers will give you the added benefit of a back up if the other charger fails. In bulk mode, what you'd be doing mostly with genset charging, both chargers will be pumping out to max acceptance of the bank or their limit. Need more, you can go to three.
Sizing can be a personal preference often based on how "quickly" you need to replenish the bank or necessitated by the charger you choose and its abilities. Deep cycle batteries like to be slow charged, so if your alternator is large, then it may make sense to have a smaller 120V charger for good deep slow charges of your bank. With boats we often don't have a choice but to "fast charge" our banks, especially if off cruising.
If your vessel is at a dock for long periods of time, or your boat is used weekends only, the added cost and size of a large charger, and the associated wiring, are often wasted if not really needed. Always check with your charger manufacturer to see how long the charger you're considering can be run at full output, especially if sizing on the small side of the charge/time equation. Some won't tolerate full output for very long, and others will barely feel it.