In #338 the specific example was a cap measuring 1cm x 1cm area. The characteristic length L is 1cm. Naturally for some different geometry a different L would apply, and in certain situations that might be somewhat arbitrary (e.g. a spherical capacitor). In general L is to be referenced wrt direction of relative motion, and of course normal to applied E. A rectangular capacitor, out of the basic simplicity of the situation there, was an obvious model to work from re gedanken experiment. I should accept some blame for the ongoing confusion in that a diagram no doubt would likely have ended any doubts about what references to what. This has taken a very windy path, yes?