How do I determine the size needed?

Is there any information on the old tank? I assume were talking about a water well.

Why not simply replace with the same size tank?

Don't know if there is a specific rule for private residential and if there is, it probably varies from location to location.

In setting up mobile home park many years ago, I was required to provide 5 gallons per minute for a 20 minute period.

That means if your well and pump provide 3 gallons per minute or 60 gallons, you tank must have a 40 gallon capacity. That seems like a lot, but if you are taking a long shower, someone flushes the toilet a couple of times and the clothes washer is running you are still not going to run out of water.

A galvanized pressure tank has a capacity of 1/3 its size. A bladder tank is about 1/2 its size.

I believe in big tanks. The bigger the tank the longer the pump last.

That mobile home park well pump is pushing 30 years.