Exchange Team Blog released great script to get transaction logs statistics.
With this script you can easily plan deploying your Exchange Servers.
As a output of this script you will get information about transaction logs which where generated during specified time.
Now you can determine how many transaction logs are generated on an hourly basis. This helps figure out how much bandwidth will be required when replicating database copies between sites, and what the effects will be of adding additional database copies to the solution. If designing an Exchange solution using the Exchange Server Role Requirements Calculator, the percent of logs generated per hour is an optional input field.