テーブルのコラムサイズを調べる ref: http://qiita.com/songfei1983/items/8a9b1fba0ae0513dcd45

size.sql

select
    sum(pg_column_size(the_text_column)) as total_size,
    avg(pg_column_size(the_text_column)) as average_size,
    sum(pg_column_size(the_text_column)) * 100.0 / pg_relation_size('t') as percentage
from t;