The size of the Oracle installation itself will dwarf the actual size of the database and the OpenACS code.
The initial database will be less than 100MB I think, and the OpenACs files will take up about 60MB on disk (I am referring to unpacking the tar.gz or .zip file which has all the SQL definitions, .tcl and .adp files, etc.)
You will want to budget some more space for log files, database dumps, etc. as well.