In hardmode, how large of a gap can hallow/corruption cross?
Corruption, Crimson, and Hallow can jump up to three blocks to affect other blocks, and Corruption and Crimson can spread farther by using vines. You can prevent them from doing so by digging a 4 block wide channel. You may be able to leave it empty, but if you have trouble with corrupted vines, you may need to stop them with blocks that cannot be converted, such as Ash, Bricks, or Wood.