What size does the wings of a dragon have to be for it to fly?

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That really depends on the overall weight of the creature. I'll have to dig through numbers of different flying things to make a guess. But I would say a large wing is needed. They may be soaring creatures rather than flapping creatures. So they would need a large amount of surface area on the wings to add more lift.

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The victories from those skilled in warfare are not considered of great wisdom or courage, because their victories have no miscalculations
- SUN-TZU

Wikipedia is your friend here:

http://en.wikipedia.org/wiki/Quetzalcoatlus
http://en.wikipedia.org/wiki/Pterosaur_size

This should tell you about the upper limits of animal flight.

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Formerly DragonRider. Almost teenage me could have been more imaginative with names.