How do you estimate the ration of the torso of a bird to its wings? A bird in my world, the aquatic roc, is essentially a collasal pelican, big enough to scoop an average galleon, a ship of 160 feet. So that means the bird has to be at least 320 feet. How do I find the size of its wings?
Kori bustard the largest extant bird that can fly.
Argentavis magnificens the largest bird ever that could fly.
Quetzalcoatlus the largest flying animal ever.
But when it flew, the Earth spun a little faster (so gravity was effectively (ever so slightly) less), the atmosphere was also thicker (making flying easter), & the atmosphere had a higher oxygen content (making muscles more efficient) ... so Quetzalcoatlus might not be able to fly today.
If you use Quetzalcoatlus as a model for your giant pelican roc, it is big enough to scoop up a normal person but won't be able to scoop up even an ordinary rowboat of the sort that seats 4 to 6 people.
In short, such a large bird is impossible in the real world, which means 'It's magic' is your only possible explanation for such a large bird. If that's what you're going with, then you can do whatever you want: slap a pair of normal size hummingbird wings on it if you want, & you're good to go.
If you just want scale up an ordinary pelican & what you're asking for is their dimensions, then you want a wingspan of perhaps a bit more than 150% of its body length.
A normal Pelican is "Length 1.3–1.8 m (4.3–5.9 ft), wingspan 2.44–2.9 m (8.0–9.5 ft)"
You talk about it scooping up a galleon, presumably this is in it's beak the way a pelican catches fish?
In which case it's perhaps worth noting that the beak is maybe only a third of the bird’s body length.