Just to load the files I believe is around 3MB. Once the board is running, threads will create additional files that will start to buildup. I'd say for a reasonable board you should try to have around 20MB of space available to allow the board to grow.

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Doug
darksaber.hispeed.com
One Ring to rule them all, One Ring to find them,
One Ring to bring them all and in the darkness bind them
In the Land of Mordor where the Shadows lie.