From what I've been able to scope out with Google searches, many sources (including Microsoft) say that a "large" SharePoint farm is one with content data in excess of 5TB. I still think that it would be interesting to get a few more responses from people on this forum as to the size of their SharePoint farm so that we can see what the distribution is.
I am still of the opinion that if you have a farm that is greater than 5TB you are going to encounter significant difficulty in performing backup and recovery. A backup will be done using a single policy (using the SharePoint agent) of the entire farm which means going through a single media server. So, even if you have multiple back-end SQL servers and get one backup stream per SQL server, your bottleneck will be the network coming into and going out of the media server that will be performing the backup.