While @gerrit may very much doubt it the periodic reflections from the original Iridium satellites, known as Iridium flares can be as bright as magnitude -8 and certainly can be as bright as Venus more frequently than that.
Venus can be seen in the daytime if you know where to look of course.
And of course if a satellite's orbit goes low enough to begin reentry, it will generate a fireball which could potentially be viewable in the day, if it was large (e.g. Skylab, ISS, Tiangong-2, Mir...) Of course then it will cease to be a satellite, and cease to be in orbit.
It's hard to state a particular height when an object will be visible during the day. A 747 jumbo jet at 40,000 feet (about 12 km) something definitely not in orbit is already difficult if not impossible to spot unless it is making condensation trails or it has some peculiar reflection.
An extremely tiny dot of light on a black background is visible even if unresolved, but an unresolved dot of black on a bright background is just not detectable with human vision. This is a combination of the nonlinear way the human eye and visual system works, and shot noise (or whatever the biological equivalent is), but that's a different question.