Largest Heater Core

If space or money was not an issue what is the biggest heater core you can get?

I would say a car radiator.

That's only assuming you are using "heater core" as a synonym for a radiator and not actually referring to the pipe matrix used as a heater.

If it is a heater core, then I don't think there's generally a lot of size variaton, although you could splice several together.

Well if money wasnt an issue you could get one custom made.