The PostgreSQL manual states:

> the longest possible character string that can be stored is about 1 GB.

· https://www.postgresql.org/docs/current/datatype-character.h...

See also: https://www.postgresql.org/docs/current/limits.html

Where do you have the 64KB number from?

A small test:

    test=# create table test_table (test_field text);
    CREATE TABLE
    test=# insert into test_table select string_agg('x', '') from generate_series(1, 128*1024);
    INSERT 0 1
    test=# select length(test_field) from test_table;
     length 
    --------
     131072
    (1 row)