Wasn't even aware of the setting, and mine is on the default of 'best display size' that according to Flickr is currently a maximum of 2048 pixels for free accounts, and 6144 pixels for Pro accounts (I have a Pro account).
Initially, I downsized images to 1600 pixels (longest dimension). Some time back, I started using 2400 pixels.
Note that even without Save As from the contextual menu, nothing would prevent someone from taking a screenshot. On the ultrawide display I'm sitting in front of, a full screenshot is 6720 × 2834 pixels (which is still only 12 MP for a 3:2 image).