No potentiometer will control a 5000W device.
You will need a controler rated for 5000W. A kind of Dimmer could work, although it will create a lot of noise on the mains power. Since a heater is usually rather slow in heating up and cooling down, a simple two-position controller will be better suited.
Your question is evidence of a certain lack of understanding of electronics. I strongly recommend you do not attempt to build such a controller on your own. Buy a commercial controller or at the very least get the help of someone experienced in that kind of circuit. This is dangerous!