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 oraclexeapporacleproduct10.2.0serverBIN

>expdp SYSTEM/globalguideline 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 oraclexeapporacleadminXEdpdump.