I say think about this stuff ahead of time and be ready for challenges like this.

Tell the customer that when TVs were invented, the advertising liars were called on to sell them, and since the diagonal is the largest dimension you can measure on the flattish surface of the screen, they chose that. (Perimeter would have been much more fun, don't you think?) The key here is that everyone does it the same way so it's comparable.  Almost everyone, that is: US TV sizes differ from Canadian TV size designations and always have.  I think we round up and they round down, sorta Texas vs. Newfoundland psychology.

Similarly, speakers have been sized the same way for at least 75 years and his anger and suspicion are misplaced. He doesn't expect a 27" TV to be 27" wide or 27" high -- nor the image that size -- so why does he assume he'll know the universal manner with which a speaker size is designated? Chill, man!