Indirect Tank Sizing
Indirect water heaters are sized by the total hot water use required. If you have one very large draw like a hot tub, it may be more beneficial to size that on a separate hot water appliance. Once you determine the hot water gallons needed compare the tanks first hour rating and boiler size.
This is for residential applications only.
When you do all the math compare it to the first hour rating of the chosen indirect.
Now chose the tank size from the manufacturers indirect chart
The choice would be an MS2-030 in this example. This should work on any indirect water heater tank with an internal coil.