Look up a thing called wing loading, the usual for birds is 1-20kg/m3 but it can be as big as 25kg/m2 (which is considered the upper limit for bird flight), in more imperial measurements the upper limit is 5.1lbs/squft and the more usual measurements along the lines of 0.20–4.10
lbs/squft. It needn't work out 100% if you can't manage to get it quite to the right proportionality but it should probably not be too of if you're aiming for realism.
You figure out the square by drawing a picture that's as exact as possible and then using the height of a character as a guide (percentages can come in handy especially with metric measurements, just take the equivalent of on metre in your picture as 100% and then calculate how much percent your measurements are, the percentage is equivalent to the cm) and proceed as you were taught in school (it can be annoying and take a long time with unusual wing forms and don't forget to take both wings into account).
Then you get the approximate weight of your character (BMI calculators are your friend,try and take into account anatomical changes that may change it up like long tails or said wings or down like light bones, huge lungs and airsacs, also don't forget about organs when making characters, they are important) and divide it through the surface area of the wing (m2 or feet squared, depending on your measurement) and you have it.
If it's really off you may need to shift/shuffle things around a bit if you're going for realism/semi-realism and not very cartoony.
There's also a quite detailed crash course (thehehe~) over here and a few things on wing type and such, it's technically a blog for dragons but angels-being six limbed, fantastical creatures that we need to get airborne somehow-are close enough for all that information to still be relevant.f-nodragonart.tumblr.com/post/…
Some other potentially useful stuff for working with your angels.f-nodragonart.tumblr.com/tagge…f-nodragonart.tumblr.com/post/…
(the lower part because wing shapes)
For folding, same thing, measure and fold, to make sure all sections stay the same length (needn't be as exact as with the calculations so just use rule of thumb). Also, look at how wings overlap when folded and how they fold (joints!!!). I have a few good tutorials I could pull up.
A very good and clear explaination:
Possibly not quite as clear but making a good point about posing.