What is the average amount of hard drive space used on a Business PC?
What is the average amount of hard drive space used on a network by a
Standard Business User?
What is the average amount of disk space used by a Business USER's
Outlook or .pst file?

it depends on what size business you are talking about
ive only been working with small businesses and ive found on average

server size is 40-400GB
business user network folder size would be a few hundred megabytes (excluding .pst)
the user's PST file will go upto 700mb before they call me to reduce
the file size (outlook only supports 2GB, but usually crashes
frequently at around 700mb or more)

server size should be greater than 100 GB
n/w shared size should be 40 GB and above
for .psd minimum 1GB required