"Buffer" is such a generic term I'm not entirely sure which part of Wowza they're referring to, as there are a number of places that certain buffer values are set. From a general tuning point of view you can define send and receive buffers as explained here. Also, each application has a property defined at the top of the Application.xml file like so
However, they may be referring to something else (e.g. DVR recording Windows size), so I'd seek some clarification on what they mean by that.