The white vents are in the viv he is in at the moment, they are there for airflow, lets cool airnin through the bottom and warm air out through the top, otherwise it would just get too hot, it also prevent high humidity levels as if the warm air cant escape it will condensate on the top and cause a lot of moisture.
Sorry about the size of the pics, not sure how to change the size of them lol
As for the viv size, as a general rule of thumb, the snake should be the full length of the front plus one side of the viv. So if you have a 36"x18" this will be adequate for a corn upto 54" long (4ft6in) and provides 4.5 sqaure foot of floor space
My corn is 61" long (5ft1in) and is in a 36"x18" viv, which is what he came in when i got him 2 weeks ago.
The cupboard i am converting measures 42"x18" so is good for a corn upto 60" (5ft) and provides 5.25 square foot of floor space
To allow for a little extra space, i am making a 'two level' enclosure, the upper level will provide an additional 1.2 sqaure foot of floor space.
So the viv i am making is plenty big enough for any corn really. They can get to 6ft but most stop growing between 4 and 5 ft.
I have allowed for upto 6 ft as he is already a big boy and they never stop growing, their growth rate just slows.
But be advised, putting a corn snake in a viv too large can cause stress.
If you have a 4ft corn, the maximum length viv you want is 4 ft really.