ESTIMATE_ONLY parameter can be used in EXPDP command, to estimate the disk space required for the export job, without doing the actual export. So when we are not sure what will be the export dump size, then better to use this option first, before generating the export dump.

By default ESTIMATE_OPTION=N

DEMO:

PARFILE WITH ESTIMATE OPTION:

cat exp_estimate.par

logfile=dbaclass.log
directory=EXPDIR
tables=DBACLASS.OBJECT_LIST
ESTIMATE_ONLY=Y

EXPDP job:

 expdp parfile=exp_estimate.par

Export: Release 12.1.0.2.0 - Production on Mon Nov 19 12:49:59 2018

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Username: / as sysdba

Connected to: Oracle Database 12c Enterprise Edition Release 12.1.0.2.0 - 64bit Production
With the Partitioning, OLAP, Advanced Analytics and Real Application Testing options
Starting "SYS"."SYS_EXPORT_TABLE_01":  /******** AS SYSDBA parfile=exp_estimate.par
Estimate in progress using BLOCKS method...
Processing object type TABLE_EXPORT/TABLE/TABLE_DATA
.  estimated "DBACLASS"."OBJECT_LIST"                       29 MB ---- >>>>>>>>>>> 
Total estimation using BLOCKS method: 29 MB
Job "SYS"."SYS_EXPORT_TABLE_01" successfully completed at Mon Nov 19 12:50:03 2018 elapsed 0 00:00:02

As it shows the estimation is 29 MB.