The number of servers and amount of storage mean nothing in my opinion.  The
issues of what type of storage it is, how many files on the servers, what
type of workload the server is, what type of server it is, what type of
network connection the server has, what is the backup window, and what
bandwidth is in the network are more interesting issues in the sizing.  If
they are all Gigabit, SAN connected ESS disk, and high end servers then the
answer is much easier to calculate.

This is not something you just throw into a calculator and bing comes the
answer.  You have to model the environment, categorize the data and servers,
figure out the critical path, and then you have a hope of configuring a
solution.