Hiya Deedee71,
Usually they say if the snake is longer than one side and the back of the enclosure then its to small but yours should be fine for now altho i would consider getting a 4ft by 18inch by 18inch if your cornsnake grows to about (4 1/2ft) - (5ft)
Just think how you would feel if you you were the snake and didnt have room to stretch out once in a while
For example my rexy is (4,1/2 ft) - 5ft now she is in a 5ft wide 3ft tall and 16inch Deep enclosure (its this size due to breeding etc...) but a 4ftx18inchx18inch would be fine for rexy.