What is the meaning of the segment name of a domain index?
The Oracle Text index consists of four basic tables, referred to as the $I, $K, $N and $R tables respectively. Other tables are used when particular indexing options are turned on. The tables exist within the schema of the text index owner, and have names concatenated from DR$, the name of the index, and the suffix (e.g. $I)


Oracle Text Whitepaper

According to the whitepaper, these are tables used by Oracle Text.

Use the following query to get the index size. For example, I have an index called I_ICOL1.

SQL&gt; select segment_name,bytes/1024/1024 MB from dba_segments where SEGMENT_NAME='I_ICOL1';

SEGMENT_NAME        MB
------------        ----------
I_ICOL1             .4375