"Please help me decide what size my house should have had and are there codes that state what size is required? "

A heating load calculation is performed (AKA 'Manual J') and then the system is designed to match the load calculated.

Radiant heat typically needs a high capacity HWH. They are far more expensive than typical HWH. It sounds like someone put in a cheap option that really shouldn't have been allowed by code.

Getting a manual J is important but maybe not something you want to bother doing yourself.

A boiler is what is used with hot water radiant heat, not a hot water heater. You can fudge with using a hot water heater and a recirculation pump if it's for a small room. If it's for a large space, the heater runs all the time and it's not designed to do that. It's not surprising that you have corrosion. You are using it way past it's design limit. You need to drain your system, fill it with filtered water, and have a boiler installed.

Thanks for the responses - The hot water tank for the radiant heat does say "high capacity". I had the heat on this week-end bumped it up to 71 and yes, it seemed to run all the time.
My house is brand new - how can I prove to my builder that what he installed isn't what should have been used.
Where would I get a "J Manual". Would it clearly spell out what size and type of tank I need.
I doubt that I can now afford to put in a boiler, if that's what is necessary for my space, shouldn't this be my builders responsibility?