You can't make a cornsnake a giant. But, by feeding it a lot you can help it to grow very rapidly. Snakes fed very frequently will grow and mature quicker that those fed less but this "powerfeeding" may be unhealthy for them.
But no matter how much you feed them you won't necessarily make a giant because size is determined by something called Multifactorial Polygenic Traits.
Multifactorial traits are determined by interactions between a gene or genes and the environment.
Polygenic traits are determined by more than one gene, with each gene providing a small, but additive effect.
Polygenic traits are continuously varying and produce a continuum of phenotypes. Individual genes of a polygenic trait follow Mendel's laws, but together do not produce Mendelian ratios.
Basically, how large your cornsnake will grow is determined by both the genetics of it's family tree and its environment. In this case its environment is created by you...such as how much it is fed, caging and other basic care. With proper care aimed towards large size your limiting factor will be the genes the cornsnake inherited.