First we need to get the name of the CAPTURE process and the actual value and then start tuning it:

col capture_name format a30
col parameter format a50
col value format a50
col set_by_user format a3
select capture_name,parameter, value,set_by_user from Dba_Capture_Parameters where capture_name like upper('&capture_name');
BEGIN
dbms_capture_adm.set_parameter(capture_name => '&capture_name',
                              parameter  => '_SGA_SIZE',
                              VALUE      => '300');
END;
/