2TB is really enough IMO. Any bigger and people start using it as permanent storage and that's not what it's meant for. I'm even finding a 1TB box is plenty but then I'm really the only one that uses it and don't record that much.
The problem is there is a lot of films that is no longer listed on Sky to download or listed that are still on my boxes that I like to rewatch. If I did as it suggests, I wouldn't be able to view them again. Some of the films on my boxes hasn't been in Sky's catalogue for years.