Evenual.
I have an iPhone 7 but I also have a Creative Cloud subscription. I don't think that makes a difference with regard to saved image sizes, but it might.
Sure. I'm happy to test on my iPad Pro tomorrow. I've also dropped a note to the product team to see what they say about image sizes... for some reason, I've never been asked, so it's not something I know.
I'll post again tomorrow.
Sue.
Evenual.
Tested on my iPad Pro and again on my iPhone this morning... both images saved at 2828 x 2828 at 72dpi.
I also checked with the product team about whether have a CC subscription made a difference in image size when saving to the camera roll. It doesn't. But... the images may be saving a bit smaller (although it doesn't sound like too much) on your 5s. I'm waiting to hear back about that.
I hope that helps.
Sue.