Yes, the selection in this case would be natural selection because whales of maximum length are mating naturally on their own choice. We are not maintaining and experimenting on them for the analysis that what will be the size of their offspring, if we do that, that is artificial selection.
Why it is natural selection?
Natural selection is the process where organisms with favorable traits are more likely to reproduce. In doing so, they pass on these traits to the next generation. Over time this process allows organisms to adapt to their environment.
Here both whales are of good size (favorable trait) and have definitely better chances of mating and producing fertile generation.
