How about collecting information about the sizes that sell most often and whether they are credit sales or subscription sales. This information would benefit many people as sometimes cropped shots sell better. But if larger photos are the higher demand then we could avoid cropping much. Sometimes compact camera photos can be sharper and better when reduced. I'm sure people with >200 sales can provide a good idea of what size sells most often. Thanks!
55-250mm standard lenses. Dual tube macro flash and external speedlit...
I don't think averages are very helpful in this regard. I think that the content and end use dictates the size buyers will want to buy. Bloggers will never need to buy anything over web size as they are prohibited from posting it untouched on the web larger than 800px wide. Illustrations will tend to sell in larger sizes if they are sets, I would imagine.
I find little relation between the camera/resolution used and the size sold. Much larger correlation between how the photo will be used and the size purchased.
I think most buyers want as much resolution as possible to be available, but will take a lower resolution shot if the content is better.
That said, I would be interested to know how often searches are performed using the image properties resolution option :)
And, if you are a "power" buyer who buys lots of images (a company account) and purchase a lot of credits at a discount I would think you might buy images in the largest sizes, keep them for later, possible use and then resize them as needed. We forget that (hopefully) there are buyers who purchase many images at one time or over a short period of time. I'm hoping those kinds of buyers outnumber the "one image here and there" buyers.
I see DT have the few standard sizes available for download. Maybe DT can charge a few cents more or at a different price if buyers want a specific resolution or size that is not in the standard range DT offered? say if DT sells at 250x250MP but a buyer need it at say 150x150MP for lots of photos we can charge a few cents more for such adjustments so that buyers need not have to adjust them on their own.
50mm F/1.8G, third party speedlights, 2 x 25mm extension tubes, Nikon...