Simply slicing the string seemed to work at first:
summary = entry.summary[0...255]
But soon we started obtaining PGError (incomplete multibyte character) when trying to persist our records in the DB. It seemed that the summary we were obtaining had a multibyte char in that position, and slicing the string seemed to slice the char itself.
Luckily, we were working on rails, and there is a helpful TextHelper truncate function to perform precisely the task we wanted.
include ActionView::Helpers::TextHelper summary = truncate(entry.summary, :length => 255)
As a side note, trying to save the same sliced string in MySql, produced no errors whatsoever. Apparently, PostgreSQL is stricter than its counterpart.