buy a book to make sure you can keep it properly,
most milk snakes are coral mimics,
and can be anywhere from 75cm (like the mexican kingsnake) too 1.8m (like the honduran milk snake.
The milks and kings that mimic coral I can think of are...
the Sonoran mountain kingsnake (Lampropeltis pyromelana), the Pueblan milk snake (L. triangulum campbelli) the Hounduran milk snake (L. t. hondurensis) the sinaloan milk snake (L. t. sinaloae) and theCalifornian mountain kingsnake (L. zonata) i'm sure there are many others that i dont know.
Also a six foot snake is quite manageable and can be kept in a 3.5' tank quite happly.
p.s my kingsnakes would eat a snake under 1m for breakfast