The Linux distribution you install will give some indication of how much disk space you need for various configurations. Programs installed separately may also do the same. This will help you plan your disk space usage, but you should prepare for the future and reserve some extra space for things you will notice later that you need.
The amount you need for user files depends on what your users wish to do. Most people seem to need as much space for their files as possible, but the amount they will live happily with varies a lot. Some people do only light text processing and will survive nicely with a few megabytes, others do heavy image processing and will need gigabytes.
By the way, when comparing file sizes given in kilobytes or
megabytes and
disk space given in megabytes, it can be important to know that
the two units can be different. Some disk manufacturers like
to pretend that a kilobyte is 1000 bytes and a megabyte is
1000 kilobytes, while all the rest of the computing world
uses 1024 for both factors. Therefore, my 345 MB hard disk
is really a 330 MB hard disk.
Swap space allocation is discusses in section 6.5.