By adding "maxQueueSize=500MB" to the outputs.conf file, under the [tcpout] stanza.
Change the 500MB to whatever you wish. It defaults to 500kb I believe. But be careful, because whatever you set this number to, it will consume 3x more RAM. So if you have it set to 500MB, it'll consume 1500MB of RAM.
Please this for more info (search for maxQueueSize):
http://docs.splunk.com/Documentation/Splunk/5.0.2/Admin/Outputsconf
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