Exchange 2007 sizing

Hi

We are currently running Exchange 2003 SP2 on Windows 2003 SP2 servers. We have our servers in an A/A/P Windows cluster and there about 5000 mailboxes split across the two virtual servers.

The company standard is to move to Exchange 2007. We'll be using SCC in a A/A/P setup as before.

However, we need to determine the number of storage groups, databases, hardware etc.

I know MS have a Storage Calculator. There is a great blog and details about it here: http://msexchangeteam.com/archive/2007/01/15/432207.aspx

I had some questions though I was hoping someone could help me out here with:

1. "How much mail do users send and recv a day" - how can I work this out?

2. "What is the average message size" - how can I work this out?

3. "Predict IOPS value" and "IOPS per mailbox" - we have Blackberry devices here. I know that the calculator can work out the value for us, based on the some factors, but should we gather IOPS value from our existing Exchange 2003 set up an use that?

4. Regardless of the answer to the question above, I'm interested to know what our current IOPS value is in Exchange 2003. Can someone explain how to get this?