Oh, absolutely. My line to the court was rather dramatised for effect :)

What you'd really argue is that since your penis size is not public knowledge, then no matter whether your actual penis is big or small, the writer's description has no bearing on the ability of the public to recognise the person being defamed as clearly you. Therefore, the accuracy or inaccuracy of the size described in writing can be simply dismissed as immaterial, with no need to inspect your pants for the truth.