Late to the party, but may be useful for anyone who comes looking in the future. There's no reliable way to convert from EPS to GB/day. There are many approaches to get closer in the blog I wrote here: http://blogs.splunk.com/2016/05/06/what-size-should-my-splunk-license-be/
Since the Splunk license is volume-based, you're better off using the total size of the content generated by your assets. Some events are bigger than others.
That said, the easiest way to do this is to download the free Splunk Enterprise, install it, point everything you have at it, and run it for two days. (Don't run it more than that, because then the free version will stop working.) Then look at how much volume you have indexed, and use those numbers to determine what your license ought to be.
(Of course, two days isn't a very big sample size, but it's better than nothing.)