how to set the -dim values(sample area) for genBSDF?

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I was reading the BSDF tutorial. In the first example in this file, which illustrates how to generate a BSDF XML file for Venetian blinds, it looks like this XML file is generated just for one slat, not the whole Venetian blind. I am a bit confused, why doesn't it use the actual geometry of the blind to generate BSDF? or does that means the actual sample area does not matter as long as it is representative of the system as a whole?

Any help will be really appreciated.