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.