You come up with a convincing argument explaining why no system conforming to the standard should be allowed to have an ARG_MAX limit less than 1048576, you come up with a convincing argument explaining why small memory model applications should not be allowed to run on standards-conforming systems on hardware using x86 compatible CPU architectures, you come up with convincing arguments why any other systems and features affected by your changes on those systems that currently conform to the standards should not be allowed to still be considered standard-conforming when a revision of the standard is approved that includes your desired changes, and then you file a change request for the standard at the POSIX Standards bug reporting site asking for the _POSIX_ARG_MAX limit in <limits.h> in the Base Definitions and Headers category to be changed whatever limit you think you can get the member companies of the Open Group's Base Working Group who will vote on the next revision of the Single UNIX Specification, the member countries of ISO who will vote on the next revision of ISO 9945 Standard, AND the individual members of the IEEE Standards Association who will vote on the next revision of the IEEE 1003.1 Standard will all approve.

The next revision of these three linked standards is expected to be approved sometime around 2020 or 2021. Then you will need to wait for companies who build operating systems to release updates that conform to the new standard and you will need to use one of those conforming products. Other operating systems might or might not increase the _POSIX_ARG_MAX limit even though they do not conform to other requirements of that standard. (Note that no Linux distribution has yet claimed to conform to any version of the POSIX standards.)