Just wanted to repost this from a discussion about this in my squadron's discord.
"I just checked the math. The angular resolution of the human eye is roughly 0.0003 radians. Using 17 metres for the wingspan of the mig 29 and the small angle approximation, we get a maximum distance to resolve the object of 56.67 km or 35.2 miles. Obviously atmospheric conditions will impact this ideal maximum.
That said, I've personally seen the planet Mercury with no magnification during a solar transit and that's only 0.00006 radians across, so the human eye can still see much smaller angular diameters, it just can't resolve them. For reference, that's the same angular diameter as a mig 29 at 283.3 km, or 176 miles."