To see how big all your folders are, launch Terminal and enter
du -sh ./*
du is 'disk usage,'
-s means 'just for the files named' (otherwise it'll dig through and count every subdirectory and file) and
-h means 'human readable'--it'll report in MB or GB or whatever is appropriate, rather than a gigantic number of kilobytes.
To see how much space all your users are using, use this command as an administrative user:
sudo du -sh /Users/*
You can also get a more detailed view--one level deeper--with this:
sudo du -sh /Users/*/*
127M /Users/admin/My Applications