The steward has very minimal requirements. Essentially it's a binary process that runs outside the cluster. It can run on AIX platform if needed.
It can also run on the below platforms
VCS 5.0/5.0MP3 on Solaris 10
VCS 5.0/5.0MP3 on AIX5.3.0
VCS 5.0.1 on HP-UX B.11.31
VCS 5.0/5.0MP3 on Linux RHEL4 / RHEL5 respectively
VCS 5.0 on Windows 2k3
And Steward Running on Linux took from VCS5.1 media.
So a small Windows server - 1 x CPU, 2 GB RAM and 30 GB HDD (min.) should be okay.
Steward is recommended to be the same OS platform and VCS version as the HA/DR member nodes; meaning if it’s a AIX VxSF-HA/DR 5.1 install or lower, implement the Steward on a AIX box also based on the 5.1 code.