+1 to John's point and your initial troubleshooting approach.
Try adjusting the Web.config of the Session Preview (OData) webservice again--maybe double it until it works and/or confirm how much content (and maybe multimedia) is used by that page. Or try a smaller page. :-)
This StackOverflow post points out two settingsStackOverflow post points out two settings, one for ASP.NET and another for IIS 7; multiple users also point out the difference in measurement unit (KB for .NET, but bytes for the IIS setting).
http://msdn.microsoft.com/en-us/library/e1f13641(v=vs.100).aspx
<configuration>
<system.web>
<httpRuntime maxRequestLength="size-in-KB" />
</system.web>
</configuration>
But also:
http://msdn.microsoft.com/en-us/library/ms689462%28VS.90%29.aspx
<system.webServer>
<security>
<requestFiltering>
<requestLimits maxAllowedContentLength="size-in-bytes" />
</requestFiltering>
</security>
</system.webServer>
Maybe someone else can fill in the details of what gets sent to Session Preview storage. I'm guessing it's the session-specific page, component presentations, and multimedia but haven't checked to be sure.