The size of the tablespace doesn't change when I import a smaller dump or deleting old data i wanna free some disc space used by an oracle database ,

I tried deleting  old data and importing smaller dump but the size of tablespace/datafile

remain unchanged.
Tablespace size is not changed by creating, dropping or importing tables (if we forget autoextend option). If your tablespace has no data after droping tables, it is easy to resize datafiles or recreate tablespace. If not empty tablespace, after tables drop, you can try to resize datafiles ALTER DATABASE DATAFILE 'file1' RESIZE 20000m.  Note that resizing may not always be done if your tablespace still contain some tables/indexes. In this case you will need to do some other actions before resizing... (alter table move or drop+import or...)