I have previously written about ways of identifying directories that take up most space using command line. There are a number of ways you can pipe commands to find out directories large directories.
Today, however, I’d like to discuss a useful little program with an ncurses frontend, ncdu. Not only does it display directories in your chosen order (ascending/descending, by size/name), but also lets you interactively browse (and be careful: delete) them. To make sure you don’t delete something by accident, you can run the program in the read-only mode calling it with the -r flag.
Apart from ordering the files/directories by size or name, there are also a number of interesting display options like showing percentage or graph, showing/hiding hidden or excluded files and directories (you can exclude certain directories or files if you want.)
Although it is not part of the stock Slackware, it can be easily installed from slackbuilds.org (it does not have any additional dependencies.)