Figure out your peak demand in gallons per minute, and find a Superstore tank that will meet the need. Do you recycle the water? If so the temp of the water going back to the heater will be a bit higher, which will help out. The other thing you could do is have a 100 gallon hot water tank (well insulated) that is kept awfully hot (say 180) with a circulator pump and a tempering valve to knock the temperature down to something less than lethal. Now there is always a huge amount ready to go, and the boiler should be able to keep up even under the heaviest demand.