You need enough space behind the wall (I assume that’s where you’re going to put it) to allow the laundry to drop freely. Optimally, it should drop near the washer downstairs. If you have kids, it needs to be small enough that they can’t stuff each other in it. Or the cat. Ok, if you can’t stuff in a cat, it’s too small, unless your cat is a puma, but kids are a big consideration. It’s really fascinating to use a laundry chute. My grandmother had one when I was a kid, but I swear it wasn’t me shoving the cat in!