I went from an iPad 3 to a retina mini, and it was definitely the right decision. So much lighter, and the screen didn't seem that much smaller. Then I got an iPhone 6, which made the mini seem slightly redundant - although I took my mini out of the house, which I never did with the 3, it's still to big to go in my pockets. My iPhone is always with me, and almost as good as the mini for watching videos - you'd think it wouldn't be, but videos play full screen on the iPhone, and letterboxed on the iPad, so actually it's much the same.
For reading I prefer my iPhone; even easier one handed, and lying on my side in bed even the mini is too big.
I gave the mini to my sister, and bought an Air 2 - it felt barely any less portable than the mini, but with a larger screen making it more useful for desktop websites and pdf-type magazines, and videos are plenty big enough.
For Christmas, I got an iPad pro!
I thought it was too big, and there was no need for it, until I tried it. It's a little awkward to hold, but the new keyboard is fantastic, so I can type much faster. It's even better for magazines from apps like Exactly, which are basically a pdf of a print magazine. For the first time, I'm able to read without zooming, and that makes a bigger difference than I thought.
Choice is good, but too much choice can lead to paralysis!
Every size is right for somebody, probably everybody at certain times. I have a dream - I'm not sure if I saw it somewhere or if I made it up (probably the former, I'm not terribly creative) - of a device like a pen, which unrolls into a phone, which unrolls into a tablet. Even though we're living in the future, it seems the future is always too far away!