Peter,
Postgres offers no 'alter table' statement to increase the column size of a varchar. However you can manipulate the system tables directly. The size of a varchar is stored in pg_attribute as the actual size + 4.
Here is an example of how I recently increased the size of a custom column called headerstuff in the ec_categories table to 500 characters:
update pg_attribute set atttypmod = 504
where attrelid = ( select oid from pg_class where relname = 'ec_categories' )
and attname = 'headerstuff';
/Bart
