Change how many 'recent' files are shown in Preview's "Open Recent..." menu

I cannot find anything in the preferences to do this, but I would like to be able to lengthen the list that Preview offers in the "Open Recent" menu. Right now it displays the 9 most recent files. I would like it to show at least 20 or so. Is there some secret way to hack this from the command line, or is there a setting somewhere that I just haven't been able to find?