total size

I may know a tiny bit about browsers but it's clear that I still know zilch about Linux. What, pray, is the command line -- or, gulp, even GUI -- way to get the total size of everything in a given directory tree? "size directoryname" of course tells me "directoryname: Is a directory"; while "ls -R -l" within the directory gives me a list of figures whereas I'd like a total. (I realize that because of cluster sizes -- or whatever the analogous term is in Linux/Reiserese -- there may be more than one total. I'll settle for any total.) With thanks and apologies for ignorance.