I have had people ask if a book is a standard paperback, and replied that the dimensions are clearly shown in the listing.

Several have replied that 18 x 11cm doesn't mean anything and I am obviously unprofessional. I generally replied with something along the lines of cheap rulers not costing much and they can then compare their existing books (if any) with the ones listed and determine the answer for themselves.

Then block them.

I have modified that response over time to 'Yes' or 'No'.