Hi Santiago,
sizing of BIG-IP Appliances may be quite difficult and depends on the needs of the customer. For sizing an Appliance you should know
* necessary Modules and amount of Modules provisioned on a single Appliance
* expected amount of Requests and connections per second (L4/L7)
* amount of SSL traffic (SSL TPS)
* necessary amount of network ports
* throughput
* Types of applications served by the BIG-IP
* Virtual or physical
A good help to find the right Appliance is the datasheet: http://www.f5.com/pdf/products/big-ip-platforms-datasheet.pdf
As far as I know there is no tool available. You can always contact your Account Manager or FSE to support you with sizing.
I know, that this was not exactly what you were looking for, but hopes that this will help you even so.
Greets,
svs