I would like to remove unneeded packages. I have already removed large files (using k4dirstat) and the only left bit is a few hundreds of packages I installed but I do not use most of them (the install, without /home, takes 15GB). 

It is possible to remove a package with dependencies individually using 'apt-get purge', and to see space that would be freed. However, this way to see space a package occupies is slow and manual, which is a problem.

How do I display packages with the disk space purging them would free under Debian?