Do data checksums apply to large objects?
If enabled, checksums will be calculated for each data page.
you can read more about check some  in the following site: http://michael.otacoo.com/postgresql-2/postgres-9-3-feature-highlight-data-checksums/

or the following website also tell about checksum http://www.postgresql.org/docs/9.3/static/app-initdb.html#APP-INITDB-DATA-CHECKSUMS 
Use checksums on data pages to help detect corruption by the I/O system that would otherwise be silent. Enabling checksums may incur a noticeable performance penalty. This option can only be set during initialization, and cannot be changed later. If set, checksums are calculated for all objects, in all databases.