The following is from the heads-up dept.
In case you haven't heard, RHEL 8 was released a few days ago and building CentOS 8 has started. You can read more about the process from https://wiki.centos.org/About/Building_8 if you are interested.
While it's way too early to give you any exact numbers, it is safe to say that you will need more disk space for mirroring CentOS when CentOS 8 gets released. At the moment 6.10 takes around 57 GB, 7.6.1810 takes around 96 GB, and 8 will probably be in the same ballpark as 7, if not more. CentOS 6 will still be on mirrors until its EOL in November 2020.
Note that disk space requirements will grow over time with the updates and content produced by the SIGs. Also note that at point release times (such as 7.6.1810 -> 7.7.19xx) both the old and new point releases will be kept on mirrors for a few weeks, temporarily increasing the disk space requirements. The same concepts apply to mirroring AltArch.
You are free to do your own estimates, but I would think 400 GB should be enough for mirroring CentOS 6, 7 and 8, with the temporarily increased disk space requirements during point release times, and some room for growth.
Most of you probably don't need to worry about this, but I suspect there are mirrors with limited disk space. If you have been thinking about moving your mirror to a new server with more disk space, now might be the time. CentOS 8 will be "released when it is ready", but my guess is that you will have at least a month before you start getting CentOS 8 content to your mirror.
Hope this helps!
References: http://mirror.centos.org/dir_sizes_centos http://mirror.centos.org/dir_sizes_altarch