I've tried to buy the biggest vivs I have room for. As long as you have enough hides and ground cover, I don't think there's any such thing as a viv or tank that's "too big".
I have a couple of custom vivs that are three and four feet long each, 18 inches deep and three feet high with two climbing levels. Some Corns love them and trundle all over them at regular intervals. However, others are less active and when tried in them, just spend all their time at ground level in particular hides, even after months of settling in. If you have a less active Corn, then a smaller tank or viv should be absolutely fine. They're all individuals.
Having said that, I wouldn't want to use one smaller than 3 feet long by 18 inches wide - I'd see that as the minimum floor space requirement for mine.