Any size charger will work. Well any size less than about 100 amps as flooded cell batteries don't like to be charged at greater than 25% of their amphour rating.

A 40 amp charger will easily recharge a dead 400 amphour bank in 24 hours.

The one reason to consider a larger charger is if you are wanting to recharge on the hook with a genset. In that case the largest charger that your generator can handle (subject to the limitation noted above) will limit genset running time.

David