From experience, Intel AMT 1.0 computers I have seenhave 96k of NVRAM and all computer with Intel AMT 2.0 and beyondI have seen have 192k of NVRAM. This said, vendors can probably change this and it's generally accepted that any single application should not use more than 48k of it so that a few applications can share this space.
I would also recommand that you try to use some type of compression when placing data into 3PDS so that this space can be most efficiently.
Hope this helps,
Ylian (Intel AMT Blog)