From this site about eye optics
The visual acuity of the human eye varies considerably, depending on the person and the lighting conditions, but it is of the order of 1 arcminute – within a factor of two or so of the theoretical diffraction limit.

and this NASA site that includes apparent diameters of the planets
Venus Observational Parameters
Apparent diameter from Earth
Maximum (seconds of arc) 66.0
Minimum (seconds of arc) 9.7
Jupiter Observational Parameters
Apparent diameter from Earth
Maximum (seconds of arc) 49.0
Minimum (seconds of arc) 29.8
Mars Observational Parameters
Apparent diameter from Earth
Maximum (seconds of arc) 25.7
Minimum (seconds of arc) 3.5
Saturn Observational Parameters
Apparent diameter from Earth
Maximum (seconds of arc) 20.1
Minimum (seconds of arc) 14.5

I guess some people have sharper eyes than others, but....