Using the following PL/SQL code one can find average size of a row in a table, the following code samples the first 100 rows. It expects 2 parameters table owner and table_name.

DECLARE
l_vc2_table_owner VARCHAR2(30) := '&table_owner';
l_vc2_table_name VARCHAR2(30) := '&table_name';
/* sample number of rows */
l_nu_sample_rows NUMBER := 100;

/* loop through columns in the table */
CURSOR l_cur_columns IS
SELECT column_name, data_type FROM dba_tab_columns
WHERE owner = l_vc2_table_owner
AND table_name = l_vc2_table_name;
l_rec_columns l_cur_columns%ROWTYPE;
l_vc2_sql VARCHAR2(10000);
l_avg_row_size NUMBER(10,2);
BEGIN
l_vc2_sql := '';
OPEN l_cur_columns;
FETCH l_cur_columns INTO l_rec_columns;
/* loop through columns */
WHILE l_cur_columns%FOUND
LOOP
/* if LOB datatype use dbms_log.get_length to find length */
IF l_rec_columns.data_type = 'CLOB' OR l_rec_columns.data_type = 'BLOB' THEN
l_vc2_sql := l_vc2_sql || 'NVL(dbms_lob.getlength(' || l_rec_columns.column_name || '), 0) + 1';
ELSE
l_vc2_sql := l_vc2_sql || 'NVL(VSIZE(' || l_rec_columns.column_name || '), 0) + 1';
END IF;
FETCH l_cur_columns INTO l_rec_columns;
IF l_cur_columns%FOUND THEN
l_vc2_sql := l_vc2_sql || ' + ';
END IF;
END LOOP;
IF l_vc2_sql IS NOT NULL THEN
l_vc2_sql := 'SELECT 3 + AVG(' || l_vc2_sql || ') FROM ' || l_vc2_table_owner || '.' || l_vc2_table_name
|| ' WHERE rownum < ' || l_nu_sample_rows;
EXECUTE IMMEDIATE l_vc2_sql INTO l_avg_row_size;
dbms_output.put_line(l_vc2_table_owner || '.' || l_vc2_table_name || ' average row length: ' || l_avg_row_size);
ELSE
dbms_output.put_line('Table ' || l_vc2_table_owner || '.' || l_vc2_table_name || ' not found');
END IF;
END;
/
Output
Enter value for table_name: TEST_OBJECTS
old 3: l_vc2_table_name VARCHAR2(30) := ‘&table_name’;
new 3: l_vc2_table_name VARCHAR2(30) := ‘TEST_OBJECTS’;
SCOTT.TEST_OBJECTS average row length: 76.88

PL/SQL procedure successfully completed.