I tried importing a database dump from a SQL file and the insert failed when inserting the string Mér into a field defined as varying(3). I didn\'t
It depends what value you used when you created the database. createdb -E UNICODE creates a Unicode DB that should also accept multibyte characters and count them as one character.
You can use
psql -l
to see which encoding was used. This page has a table including information about how many bytes per character are used.