@mikes41720
First, I don’t think my attachment pages have featured images – could that be a problem in v7.4? I guess if the attachment is an image, then the image itself becomes the featured image? This works in v7.3 so it’s probably not an issue.
Second, my photos are 980 pixels wide and 800 pixels high (whichever comes first) so they’re not too large – in fact, reading the link you provided, the width may be considered too small. But then again, I never had an issue with this image size, sharing to Facebook, Twitter and Google+ regularly – and it always worked up to YS v7.3.
Third, and just in theory, assuming I WOULD upload larger images (say, 4500 x 3000 pixels), there is no way in WordPress to select a specific SIZE for the Featured Image, at all – I lack the insight as to how this works so maybe there is, but as a user, all I do is click on “Featured Image” and select one from my Media Library. There is no size selection.
Thanks for the help.