The documentation page says that there are Cause and CausePath fields available that can be used for describing the actual cause of the SFN state machine run failure.
What would be the maximum amount of characters that I can put there?
I would like to pass as much information as possible to simplify issue investigation and being able to preserve such errors in the SFN run history
The AWS quota on payload size applies to the whole thing, not to a field. The "maximum input or output size for a task, state, or execution" is "256 KB of data as a UTF-8 encoded string".
How many characters fit into the 256 KB depends on the UTF-8 encodings.