Use the dba_segment
select owner, segment_type, sum(bytes)/1024/1024/1024 as GBs from dba_segments group by owner, segment_type order by GBs desc
A little word of explanation. The size is returned in bytes. So if you want KB you divide by 1024, if you want mega bytes then again by 1024. Here we wanted Giga bytes so we divided 3 times.