Well over an exabyte for raw storage ...but mostly a lot of the "storage" is RAIDed and/or backed up and redundant. So actual "data" that you and I may download (if we had the option to download, say, every song on iTunes) is probably 1/5th to 1/10th what the actual total storage is.
It's a bit hard to calculate without any physical characteristics. It's pretty easy, however, to count drives....if each 1TB SATA drive was 5" by 7" by 2" thick, you could figure out how many would fit in, say, a 10x10 foot room...but then you need to take into consideration electricity, # of outlets available, heat, etc.
Per Wikipedia: As of 2011, no storage system has achieved one zettabyte of information. The combined space of all computer hard drives in the world does not amount to even one yottabyte, but was estimated at approximately 160 exabytes in 2006.[1] As of 2009, the entire Internet was estimated to contain close to 500 exabytes.