If Humans had wings, approximately how big would they have to big in order for us to fly around. Lets make the assumtion that the wings would more along the lines of Feathers rather than skin like bats. Would they be too large that it would be impossible to fly because humans wouldn’t be able to develope the pectoral muscle mass to power them? I’d imagine the wings would have to be fairly large giving that we are probably proportionally much heavier than birds.
We’d need bones vastly less dense than our current ones. The muscles needed to power our wings would add a lot to our weight besides the weight of the wings themselves.
Avians aren’t my strong suit, though, so I’ll take the backseat to whichever more knowledgable Doper wanders in. 
IIRC, I remember reading that for the leverage to operate them, we would need a ribcage six feet deep.
I would think that if you could alter a human enough to make it fly (even a little bit), it would be so different from the way the humans are now that you would not be able to call it a human.
A quick Socratic run-down: ever eaten chicken, specifically, a chicken breast? they can’t fly, and yet the muscles required to flap their wings ineffectually are a non-trivial percentage of their bodyweight. Ever eaten duck or goose and looked at the breast meat? That’s a flying animal’s pectoral muscle. Now look down at your pecs. Imagine that Pamela Anderson’s breasts and all the accompanying polymers are pure muscle, aligned in such a way as to make her as airworthy as possible. Imagine that, other than working on those pecs for months and eating pure protein, she’s been allowing the other unnecessary muscles to atrophy. She still couldn’t fly.
Also – we’re mammals, so we’d pretty much have to get bat wings, as much as feathers do happen to look prettier. You would need to start with the following changes:
- hollow bones
- lighter, probably narrower heads
- shorter, less muscular limbs
- narrower hips and much smaller breasts on females
- gigantic back and pectoral muscles
- a place to put the wings on our amazingly space-efficient skeleton
Start by looking at flying birds that have the same body mass as us. Note there aren’t any. Also note that large birds like albatross do much more gliding on thermals than pure muscle powered flying. They aren’t particularly graceful on takeoff or on the ground.
I should have added the scaling problem. If you take an ordinary bird as a model and scale it up it won’t fly any more. If you scaled it two times as large it would have four times the wing area but eight times as much mass giving it double its original wing loading. In aviation a high wing loading is a bad thing for making stuff fly. Scale a 6" long bird to a 6’ tall human and it has 12 times as much wing loading. The same factors apply to airplanes and big planes make it up with extremely strong structures and high airspeed to gain lift.
The largest flying creature ever was the pterosaur Quetzalcoatlus, which probably weighed around 190lbs at most. It also had a wingspan which could reach up to 39-42 feet (depedning on the individual). Like the albatross, it was a soarer, not a flapper - indeed, it proably was not capable of anything beyond very slow wingbeats. And that’s with the requisite adaptations for flight, including a deep, keeled sternum to accomodate the increased mass of pectoral muscles.
Humans, lacking the associated adapations (which necessarily go beyond simply attaching a pair of wings to the body), would likely only be capable of gliding, much as we can already do with a hang-glider, were one to strap on a set of wings. Turn the fabric & metal frame into one of flash and bone, and that’s about the best you’re likely to get in terms of “flying humans”.
Chickens can fly quite well. They can’t fly for long periods because of the type of muscle the have powering the wings, but they sure can fly. There are a few ornamentl breeds that can’t fly, but most of the world’s chickens are good flyers.
Yea, but…wouldn’t it be cool?
Yes, it’d be cool.
But OTOH, don’t forget that to gain wings you give up arms & hands. THAT would be a major bummer, essentially precluding any sort of progress beyond the pure animal stage. We might be the land equivalent of very smart dolphins, but that’s about it.
Now wings plus arms, that’d be cool. But, by definition, we’d be insects then and that’s too icky for my taste. 
I would think that there is a much simpler and more economical way to make humans more effectively flight-capable: a substantial reduction in average size. After all, “human” doesn’t necessarily have to imply “six foot tall, 180 pounds,” or whatever. As an example, Lucia Zarate was a Mexican woman who stood only about two foot tall and probably weighed something like thirteen pounds, yet was physically quite well-proportioned and by all accounts was quite intelligent and articulate. It would be much more plausible to add wings to a human of this size (or smaller, if possible) than to create a linebacker-scale Man-Bat. The only critical boundary would be how small the human brain can be scaled before intelligence is adversely affected. Granted, your end result would be not so much an angel as a cherub, but the laws of physics would thank you for it.
This is anecdotally so vague as to barely qualify for a GQ answer, but back twenty years ago or so, I read some books on universe building and the author, a fairly techy science fiction writer (* maybe * Poul Anderson) had done the math, and from an energy perspective, human flight just isn’t efficient. The wings have to be too big, pushing the structural limits of bone and sinew, and, as people have noted, you need some major body modifications to even get in the ballpark (the keeled sternum, big flight muscles, huge lung capacity, and toss everything heavy, including that big fat brain)
This isn’t to say that human powered flight is impossible, witness the Gossamer Albatross . However, look at the specs of the craft. A 90 foot wingspan and it achieved a maximum speed of 18 mph and averaged an altitude of 45 feet when pedaled by a professional athlete. A 90 foot wingspan would be pretty unflappable (not too mention fragile), and you wouldn’t exactly be perching on tree limbs and nibbling on feeders.
Basically, humans with wings would spend most of their time on the injured disabled list, rubbing liniment on their flight muscles.
Obviously a flying ‘human’ would be the biggest bird in the world. So the 2M wingspan of an albatross would just a start.
A 100 kilo person would need muscles and other structures weighing an additional X kilos, and those kilos would need even more to lift them. All in all the numbers would start to take an alarming turn.