You must run the following script to populate the File Size field for existing data.

List<ApttusDocumentVersionDetailc> documentVersionDetails = [select id, ApttusContentIdc from ApttusDocumentVersionDetailc where ApttusContentIdc != null AND (ApttusFileSizec = 0 OR ApttusFileSize_c = null) limit 1];
if(documentVersionDetails.size() > 0) {
Apttus.DocumentFileSizeUpdateBatch documentFileSizeUpdateBatch = new Apttus.DocumentFileSizeUpdateBatch();
// Execute DocumentFileSizeUpdateBatch
Database.executeBatch(documentFileSizeUpdateBatch);
}