It depends who you ask. Many care sheets recommend 20 g long to be the minimum (I saw one that said 55 g from an adoption site).
Many serious collectors and breeders use sterilite containers and I believe that the dimensions are usually smaller than a 20 g long.
I would say that if a 20 g long is what you have access to (or can afford) then you should be ok with it. As your corn gets older, you may want to consider upgrading.
Basically, 20 g long is considered the minimum for most hobbyists - decide whether you want the minimum for your corn, or whether you want to give it more room. More room can definitely have its benefits, in that your corn will have more room to exercise and climb (hopefully) and possibly less stress from feeling confined. In general a larger tank could promote a healthier lifestyle for your snake, while a 20 g long would not necessarily mean you will have an unhealthy snake.