How to find an Index size

select segment_name,segment_type,bytes/1024/1024 MB
 from dba_segments
 where segment_type='INDEX' and segment_name='your_index_name';

How to find a table size

select segment_name,segment_type,bytes/1024/1024 MB
 from dba_segments
 where segment_type='TABLE' and segment_name='your_table_name';