Scenario:
You want to build a model of protein with trans-membrane parts. You can specify the di-electric constants of water and lipid interface and you can even find a lot of parameters for you to build this kind of model. The problem is: How thick will you specify your membrane?
Discussion:
Normally the thickness of the cell membrane would be between 7 to 10 nm. I do think that it depends on the system you want to build. Is it in eukaryotic cells or prokaryotic cells? Is it in an organelle?
References (arbitrarily picked from the Internet)