private long getOptimalPartSize(long contentLengthOfSource) {

        long optimalPartSize = TransferManagerUtils
                .calculateOptimalPartSizeForCopy(copyObjectRequest,
                        configuration, contentLengthOfSource);
        log.debug("Calculated optimal part size: " + optimalPartSize);
        return optimalPartSize;
    }
Computes and returns the optimal part size for the copy operation.
aws-java-sdk-s3/src/main/java/com/amazonaws/services/s3/transfer/internal/CopyCallable.java