Hi Brenda and everyone! Hope you are having a good 2016. Yes, it's good to have a chat thread and I will try to answer some questions. As for your OP....
I have noticed this and I have sent a message to my buyers as a courtesy. One in particular was months ago and STILL says they haven't downloaded their files. I sent a message to my buyer but didn't get a response. I hope it's a glitch or error on Etsy's part because how could a buyer just walk away after buying something? Anyway, because of the nature of these kinds of sales are beholden to the technology we depend on and because of the way Etsy's case system is set up, I would highly recommend to any seller that you follow up on all digital sales, at minimum, with a message to your buyer.
Not that I've ever had a case opened up, but the thinking is that by keeping a communication 'paper trail' on Etsy, I can at least cover myself on my end if there is ever a problem.
As for use limitations, I think all sellers should be specific on what you can and cannot do with commercial graphics. I try to be clear in my listings but sometimes you just need a second pair of eyes to read your policies to make sure they make sense to the average buyer. Etsy Success team is great for this kind of feedback, or even just asking a friend or spouse.
And for your final question, yes I am struggling to keep up with the ever-changing sizes and formats on Etsy and elsewhere. Etsy just changed their banner requirements and I was left scrambling to change 25 listings to fit their new cover photo size. And the placeholder graphic size has changed. It seems like a full time job just to keep up with all these changes. When this happens, it can be embarrassing because we graphics peoples are supposed to know-all, but you fix the error and know for next time.
I think it really depends on what it is. If it's a graphic for Facebook, for example, I try to stress to my customer that it's not going to fit all social media, like twitter or a blog, so you have to make something for each site, unfortunately. But with the new cover photo on Etsy, the size is similar to Facebook and twitter. I haven't tried it out yet, but has anyone checked to see if you can use your cover photo across other sites as well?