Some people have claimed that Ivory-billed Woodpeckers are so much larger than Pileated Woodpeckers that the difference is immediately obvious and that birds in the field can be identified almost by size alone, even when flying away quickly or through the trees. So, here's a photo of an Ivory-billed Woodpecker and a Pileated Woodpecker side by side, taken at the Harvard Museum of Natural History. You tell me, does size matter?
If you think that IBWOs are still so much larger than Pileateds, then you have to answer this--even if this Pileated is "overstuffed" and the IBWO is "understuffed" here, are the size differences so great that they would be immediately obvious in the field, especially under difficult viewing conditions, at a distance, without the ability to make an actual side by side comparison?
Here's some perhaps useless "facts" that show how problematic it is to depend on size to differentiate between these species. From the Birds of North America series, overall length of the two species is almost identical:
PIWO 40-49 cm
IBWO 48-53 cm
Northern Pileateds are the bigger ones, but still, this may not be a huge difference accurately observed on distant birds in flight.
As for weight, some have argued that IBWO is a lot heavier. Here's the "difference":
PIWO 250-350 g (248-297 g, mean 273 g for southern birds)
IBWO 454-567 g
While this might look like a big difference, here's the kicker. We only have three weight measurements for IBWO: "20 oz.", "1 lb.", and "weighed upwards of 1 lb." It is almost impossible at this point to know how accurate these measurements were, how they were made, or how precise. Even if these measurements were completely accurate and precise, with only three measurements we wouldn't have much to go on to determine what the true variation in size was with the species. So, without more information, it is best to take these measures with more than a grain of salt.
If this were a normal bird identification problem, we'd have many observers with experiece with both species in the field pouring over lots of photos of side by side comparisons to really show how different the two species are, and how reliable this feature is in the field. Right now we have none of that because we have no recent photos and no side by side comparisons of living birds. As with most everything else about IBWOs, their apparent size in the field is almost completely speculative at this point. If they really are bigger in Pileated in the field, then it still remains to be worked out how that really translate into a workable field character and under what conditions. We're a long way from having that, so any reliance on impressions of size (which are notoriously unreliable on lone birds anyway) as a field mark is completely premature in discussions of possible IBWO sightings.