Karsten Palmvig has created a script for analyzing the transaction logs. The script is very helpful for sizing the Exchange organization. You can find the complete article in the Exchange Team Blog:
http://blogs.technet.com/b/exchange/archive/2014/03/07/now-available-getlogfileusage-ps1-script.aspx
So if you are about to migrate to Exchange 2013, you should take a look at the script.
