How To Estimate Disk Space Needed for an Export Job? - Oracle DBA FAQ - Loading and Exporting Data

If you just want to know how much disk space for the dump without actually exporting any data, you can use the ESTIMATE_ONLY=y parameter on the expdp command. The following tutorial exercise shows you how a system user wants to see the disk space estimates on a full database export:

>cd \oraclexe\app\oracle\product\10.2.0\server\BIN

>expdp SYSTEM/fyicenter FULL=y ESTIMATE_ONLY=y
Starting "SYSTEM"."SYS_EXPORT_FULL_01":  SYSTEM/**** FULL=y
  ESTIMATE_ONLY=y

Estimate in progress using BLOCKS method...
Processing object DATABASE_EXPORT/SCHEMA/TABLE/TABLE_DATA
.  estimated "FLOWS_020100"."WWV_FLOW_PAGE_PLUGS"      42 MB
.  estimated "FLOWS_020100"."WWV_FLOW_STEP_ITEMS"      27 MB
.  estimated "FLOWS_020100"."WWV_FLOW_STEP_PROCESSING" 16 MB
......
.  estimated "SYSTEM"."SQLPLUS_PRODUCT_PROFILE"         0 KB
.  estimated "TSMSYS"."SRS$"                            0 KB
Total estimation using BLOCKS method: 169.8 MB
Job "SYSTEM"."SYS_EXPORT_FULL_01" successfully completed

Now you know that you need 170 MB disk space to export the entire data base.

Oracle also records the screen output in a log file called export.log at \oraclexe\app\oracle\admin\XE\dpdump.