I don't think there is a clear answer, and you can see that by this old discussion over on
WUS. I think the consensus is that you measure from 8 to 2 or 10 to 4, and that way you exclude the crown, and often the crown guards too. That said, it seems to apply differently with different watches. Your watch is reported at having case size of 41.5mm, and based on your picture, that obviously includes the guards. Equally, a Speedmaster Professional is 42mm, which includes the guards, because the case is asymmetrical. The earlier non-Professional symmetrically-cased models are reported at 39mm, because there are no crown guards - yet the watch width is similar because of the dial-bezel combination.
Therein lies the problem. Irrespective of the physical size of the case, a watch can wear larger or smaller, and it has a lot to do with the colour combinations of dial and bezel, the expanse of the crystal when the bezel is very small, the length from lug to lug, and many other things that trick the eye. The issue here might be less the case width of the watch you sold, but rather the expectations of the buyer.
Is he claiming misrepresentation because he is now saying it is too small? It may be that he is suffering buyer's remorse? You can take one of two approaches: (a) argue that he should have done his research (which he should have) before he bought and the fact that he now doesn't like the watch is his problem (which is really is), or (b) take the high ground by taking the watch back, making sure he pays return postage and all associated fees and charges so you are not out of pocket, and then resell the watch to a more deserving buyer. There are pros and cons to each approach.