Could anybody estimate the number of people interested in different topics, for example, how many are interested in translation, how many are interested in web technologies, how many Python?
I usually suggest 8GB or bigger. The Debian DVD images can be just a little bit bigger than 4GB.
That may depend on the amount of free disk space, sometimes people have to remove things or they want to make a backup anyway before shrinking their Windows partition.
People can also run Debian in a virtual machine (e.g. VirtualBox) or with remote access to a server using SPICE or SSH