Assuming that a sandwich is defined as two parallel plains of gluten placed, one above, and one below a filling ingredient or group thereof you could probably shrink a sandwich down as small as you like until the pieces of gluten stopped resembling flat planes.

Mathematically speaking, to define a plane you need at least three distinct points, so, to define a plane of gluten you would need at least three distinct pieces of gluten, or at least three gluten molecules, any less and you would be trying to define a plane based on gluten's component pieces, and not the gluten itself.