I think one of my personal sites are around 2GB, but on the average sites hosted with us are around 500MB. The only site that ever exceeded 5GB was a comic site containing flash, audio, video, and very large jpgs - as well as a busy forum with plenty of uploads and attachments.

However before his site hit 10GB in size it needed a dedicated server to handle just the traffic.

I would have to agree that the average user only needs a 1GB hosting account.