If you just want to calculate the schema size without tablespace free space and indexes :
select
sum(bytes)/1024/1024 as size_in_mega,
segment_type
from
dba_segments
where
owner='<schema's owner>'
group by
segment_type;
For all schemas
select
sum(bytes)/1024/1024 as size_in_mega, owner
from
dba_segments
group by
owner;