Is there any command that could show me the size of several folders in linux, perhaps ranked from biggest to smallest?
As others said,
du is the way to go. But knowing the options to
du is essential. Here they are:
du -m --max-depth 1 /foo /bar
This will give you the size in megabytes of the directories contained in
/bar. If you want the output to be sorted, pipe it through the
du -m --max-depth 1 /foo /bar | sort -n -k 1
If you would like a graphical (X11) display, consider installing
xdiskusage. You can either pipe the output of
du into it (as you might do if you're running
du as another user, or on another system, or at another time), or you can run it interactively and it will invoke
du for itself.
As usual, once it's installed, consult the man page.