Hi Wajeeh,
Glad to see you're going down the path to virtualize Exchange 2010 on vSphere. You'll find that VMware has a ton of great resources to help you on this journey right here: http://www.vmware.com/go/exchange
It's very difficult to answer your questions without some context about your environment. When it comes to Exchange 2010 there really are no "generic" sizing guidelines. For example, there really isn't any rule of thumb to say that for 5,000 users you need xx amount of vCPU and yy amount of memory per server. You'll need to do some math from your environment to figure that out.
Thankfully Microsoft provides all of the numbers you'll need to figure out how much CPU and memory you'll need to size your servers. This link does a good job of taking you through some of the math you'll need to understand:
http://technet.microsoft.com/en-us/library/ee712771.aspx
In addition, Microsoft makes a calculator that you can use to plug in your requirements and it will produce the required amount of CPU and memory for your Exchange environment. Take a look here:
http://blogs.technet.com/b/exchange/archive/2009/11/09/3408737.aspx
To answer your other questions:
- Yes, you can combine the Hub Transport, Client Access, and Mailbox Server roles on the same virtual machine. You'll need to size for it so that you can ensure proper performance. See here for more info: http://technet.microsoft.com/en-us/library/dd298121.aspx
- The Exchange databases can either be on an RDM or on a VMDK attached to the virtual machine. It comes down to your requirements and if you're using a SAN based snapshot, backup, or replication tool. If you're not then I would stick wtih VMDK files.
- The Edge Transport server is designed to be at the network perimeter and not joined to your domain. You cannot combine it with other Exchange roles so you'll need to deploy it on a separate virtual machine.
I hope this helps. Read the Microsoft links to get an idea of how to size your environment based on your organization's requirements and that should give you a good start.
Matt
