I think he's trying to say that the size of the barn won't change, but it should appear smaller and smaller the higher up you go... Just a guess...
I predict this will not get resolved if my assumption isn't correct.
In reality, I can see barns in rural areas up to cruising alt (35-40k as previously stated)?
The barn will seem smaller, but it shouldn't appear microscopic at 6k like you're making it sound like it should be.