Any recommendations for a small form-factor, yet functional, Android phone (about the size of an iPhone 4/S)?
I have a Sony Xperia Z5 Compact. This is often recommended to people asking for a small phone, and it's a very nice bit of hardware, but it's still too big.
OP, grandparent is correct, I was in your situation and a 5X was perfect. Pixel and Pixel XL look like they'll be too big, but you should wait for the October 4th event to confirm.
Think about this fact , many of high end phone from last year or year before that (which customer spent ~700$ -which is quite money-) has not fixed stagefright vulnerability and don't have any plan to update to new version. Thats total fraud , which they pull off (manufacturers).
And that would be exactly which ones? Samsung did update their high end phones, so did Sony, HTC and LG.