If I add an additional column to handle special values, how will this impact my table size?
Change the datatype of Detail to VARCHAR(2000) or TEXT*.  This will be enough to handle your 1900 length value plus a little padding for future exceptions.  This will add no additional overhead beyond the additional storage needed for that particular row.
If you add another column you will add 1 byte to all of the rows (which isn't a big deal) but now you have to contend with two columns with different names but the same purpose.
Postgres documentation explains storage requirements in more detail:
https://www.postgresql.org/docs/current/datatype-character.html
*The TEXT datatype is not ANSI compliant, if you do use it understand the implications.