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.

Tadaaa