A smaller block size would make this a soft fork, as unupgraded nodes would
consider the new blocks valid. It would only make things that were allowed
forbidden, which is the definition of a soft fork. For a hard fork, you
need to allow something that was previously invalid.

On Tuesday, August 18, 2015, jl2012 via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:
>
> s = 1.5MB. As the 1MB cap was set 5 years ago, there is no doubt that all
> types of technology has since improved by >50%. I don't mind making it a
> bit smaller but in that case not much valuable data could be gathered and
> the second objective of this experiment may not be archived.
>