select table_name, pg_relation_size(quote_ident(table_name))
from information_schema.tables
where table_schema="public"
order by 2
This shows you the size of all tables in the schema public if you have multiple schemas, you might want to use:
select table_schema, table_name, pg_relation_size('"'||table_schema||'"."'||table_name||'"')
from information_schema.tables
order by 3
SQLFiddle example: http://sqlfiddle.com/#!15/13157/3
List of all object size functions in the manual.