how to calculate a table actual size

I read some aticle

for example ,

i create table

CREATE TABLE emp (
empno  NUMBER(4) NOT NULL,
ename  VARCHAR2(10),
job    VARCHAR2(9),
mgr    NUMBER(4),
sal    NUMBER(7,2),
deptno NUMBER(2))

tablespace test;

i  insert some value in table such as :

INSERT INTO emp VALUES
(7369, 'SMITH', 'CLERK', 7902, 800, 20);
INSERT INTO EMP VALUES
(7499, 'ALLEN', 'SALESMAN', 7698, 1600, 30);
INSERT INTO EMP VALUES
(7521, 'WARD', 'SALESMAN', 7698, 1250, 30);
INSERT INTO EMP VALUES
(7566, 'JONES', 'MANAGER', 7839, 2975, 20);
INSERT INTO EMP VALUES
(7654, 'MARTIN', 'SALESMAN', 7698, 1250, 30);
INSERT INTO EMP VALUES
(7698, 'BLAKE', 'MANAGER', 7839, 2850, 30);
INSERT INTO EMP VALUES
(7782, 'CLARK', 'MANAGER', 7839, 2450, 10);
INSERT INTO EMP VALUES
(7788, 'SCOTT', 'ANALYST', 7566, 3000, 20);
INSERT INTO EMP VALUES
(7839, 'KING', 'PRESIDENT', NULL, 5000, 10);
INSERT INTO EMP VALUES
(7844, 'TURNER', 'SALESMAN', 7698, 1500, 30);

this just is exmple, i want to kown that if i create tablespacce test size 1M;