Forcing a graphene sheet to a given size

I am simulating a membrane (a couple of MOF unit cells) and I have a graphene sheet on the feed side that acts as piston to simulate a pressure-driven process. My issue is that I want to make sure the size of the graphene sheet matches the MOF membrane so that I don’t have problems in periodic boundary conditions (extending only in x and y). Is there a way to stretch/compress the graphene sheet to match a given size? The graphene sheet will be rigid (all atoms move in unison according to the pressure gradient).

If the graphene sheet is rigid and the forces within the carbon atoms are cancelled, then the graphene sheet wont impact your box size in x and y even if you are using NPT to adjust the box size, so it should be fine.

But, does the piston have to be made of graphene? If not, its easier to use a piston made of a material with adjustable dimensions, such as for instance a frozen layer of fluid.

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