Depending on your storage you will need to specify a tempfile; also if you have a tablespace already TEMP for example you can set the DEFAULT to be the new TMP tablespace. Uniform size depends on the size of sorts you expect to occur in your temporary tablespace..
I always set auto extend off for the temporary tablespace just in case a Cartesian join goes ballistic and consumes tones of space.
CREATE TEMPORARY TABLESPACE TMP
TEMPFILE '/database/SID/data01/sid_tmp_01.dbf' SIZE 3000M AUTOEXTEND OFF
EXTENT MANAGEMENT LOCAL UNIFORM SIZE 131072K
SEGMENT SPACE MANAGEMENT MANUAL
ALTER DATABASE SID DEFAULT TEMPORARY TABLESPACE TMP