We don't really keep track of the size of forums, let alone "largest" forums (the UBBDev
team in charge of updates isn't staff of UBBCentral, so we don't really keep track of anything).
Some decent sized UBB.threads forums offhand would behttp://backofthenet.comhttp://koptalk.comhttp://divorcebusting.com
A better listing would display if you were to search a major search engine, like Google, for "Powered by UBB.threads™ PHP Forum Software"
(the notice at the bottom of each forum installation which isn't paying for debranding).