Is there any growth problem?

My brother had an iguana and it never growed they way it had to grow, I think it was because he had the iguana on a 20l tank... but... if my snake is in a 20 gal tank it will have growing problems? like not growing the size it's supposed to be?

I seriously doubt the tank was the iguana's problem, but I don't know much about lizards. The tank will not cause growing problems for a cornsnake, other than eventually it might be just a bit crowded. Adults can live in 20L's but that's the minimum recommended size.

Lizards in general are more complicated to keep than snakes. Iguanas are famous for having a Metabolic Bone Disease that is caused by nutritional problems. This is most likely what happened to your brother’s iguana. It causes deformities and death.

Lizards require ultraviolet light. They require calcium supplements, vitamin and mineral supplements. They require both vegetable matter as well as insects to eat. I think most people that buy a cheap iguana don’t know what they are getting into. Unfortunately, it’s the iguana that pays the price.

Snakes of the other hand get all the nutrition they need from their prey. In the wild they live healthy lives eating only rodents. That is easy for us to duplicate in captivity.

Oh so I won't have a problem with my snake? It will grow good? I have a friend who has a sinaloa king snake en it's like 50 cm long and she is 2 years old so I was wondering if she had a growing problem, I want my snake to grow big

Do a search for "Munson Plan" and follow that. Your snake will grow to as big as it's genetic makup will let it.