Lets indulge in a bit of Mad Science, here.
Assuming the world described here: Making the Enterprise Fly (60% of earth's gravity, 50% denser atmosphere) what is the wingspan needed to allow an average adult human to fly?
Assume feathery wings, and that the resulting winged human needs to be able to fly well, not just founder through the air technically flying.
According to google:
The largest species of bat are a few species of Pteropus (fruit bats or flying foxes) and the giant golden-crowned flying fox with a weight up to 1.6 kg (4 lb) and wingspan up to 1.7 m (5 ft 7 in).
It also tells us that
While the average body mass globally was 62 kg, North Americans weigh in at 81.9 kg.
You've said average at a gravity of .6g so we'll say use 38.
It's actually the surface area of the wing rather than the span which generates lift. Taking a big simplification we'll assume the bat has square wings. So assume our bat has an approximate wing surface area of about 2.9 square metres giving a surface area to weight ratio of 1.8.
We can scale up here... a 62kg at .6 gravity human would require a wing surface area of 68 square metres equating to a wing span of about 8.5 metres.
These are VERY rough calculations (for one I've never seen a bat with square wings) however I hope it illustrates that a human would require enormous wings in comparison to their height (not to mention that the wings would also add weight to the person which would also require extra wing!).
I've not taken into account the increased atmosphere because humans are a lot less aerodynamic than traditional flying creatures and we're not factoring in the weight of the wings.
Clearly your average human would need to lose a lot of weight for this to be even remotely possible, we're not talking about dieting... we're talking about lightweight bones, organs and muscles!
It really depends how fast you expect them to fly. But let's look at how your changed world will affect them.
The equation you're looking for is:
Area = (lift force)/(half velocity * velocity * air density * lift coefficient)
Or mathematically:
$$ A = \frac{L}{0.5 v^2 \rho C_L} $$
$A$ (area) is the number we're looking for.
$L$ (lift force) must be equal to the mass of the person, in order to support their weight. People on earth average 62kg. With all the exercise from flying, they might average a little lower in that world.
$v$ (velocity) is the take-off speed: the speed at which the forward movement through the air makes the lift cancel out the person's mass.
$\rho$ (air density) is specified as 1.5 times Earth's.
$C_L$ (lift coefficient) is approximately 1, and depends on the angle of attack and wing shape. You can assume their wings are decently well shaped, so can ignore this term. Changing this to make thrust is, basically, what flapping does.
Wing loading on an Earth hang glider is as high as 6.3 kg per square meter, and the takeoff speed is about 15 mph.
From the equation above, we can see that the wing area is proportional to mass ($L$), and inversely proportional to air density ($\rho$). In other words, wings need to be larger when there is more mass; and don't need to be as large when the air is denser.
So we can multiply the needed area by $\frac{0.6}{1.5} = 0.4$. So 6.3 kg per 0.4 m2, or 15.75 kg per square metre.
That's roughly a quarter of human bodyweight, so you'd need four square metres for an average person. Two square metres per wing. Assuming folded wings like birds have, that's certainly achievable.
Now, let's push the limits. Adults with anorexia have a BMI below 17.5. So let's aim for that, as the acceptable limit of thinness. When $M$ is mass, and $h$ is height, $ M = h^2 \times 17.5 $.
This scales with the square of height, so height is definitely not a good thing. So assume 1.5 m (approximately 4'11").
$$ 1.5^2 \times 17.5 = 39 \space\text{kg} $$
That needs only 1.25 m2 per wing!
Something else to note, though, is that lift improves with the square of the velocity, but linearly with area. So if you double the speed to 30 mph, you can quarter the area: in a 30 mph wind, you could hover in a trenchcoat.
If you halve the speed, you only need to multiply the area by $\sqrt{2}$: so for a normal weight person, you can have gliding at 7.5 mph with 2.8 m2 each side, which is still in the range of achievable, and means they could take off in a breeze or at a run, without needing to jump off a hill. For our petite skinny person, that's only 1.8 m2 per side.
Acrobatics would require higher velocities, but that's what a dive is for! :D
Humans can fly quite well with wings the size of a typical hang glider. Of course they are doing soaring flight rather than flapping, but then the largest species of birds mostly soar. (As did the even larger pterosaurs like Quetzalcoatlus.) See e.g. condors, albatrosses, etc.
Soaring puts limitations on their lifestyle and habitat. They'll need to live near the tops of cliffs or hills, and in windy regions.
The only way humans could ever fly is by being little flying angels! Let me explain:
What if you allowed some sort of genetic mutation to the human DNA so that our growth stops very early, around 4 years old.
It might have some minor impact on our cognitive functions since our skull would have smaller volume. You might solve that by also modifying our DNA so that our head is bigger in proportion to our body size. Also modify our DNA so that we have wings, and the appropriate muscle and cardio-vascular system to move them fast enough to fly.
Our body weight would then be around 15-20kg, similar to an albatros. Then flying on earth (1G) would seem possible.
