using adsiedit, go to:
Configuration > services > microsoft exchange > <orgname> > administrative groups > exchange administrative group (..) > servers > <servername> > informationstore
there, right-click information store, and edit msexcheseparamcachesizemax .
For more information on configuring this value and how to calculate which value to use, check out this article:
be advised that exchange 2010 works with 32k pages, so you need to divide by 32 in the formula. Also, end-user performance may be degraded when limiting the cache size.