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:

EXPDP job:

As it shows the estimation is 29 MB.