When the iPhone first came out 8 GB was my size, of course all I had was music.
My iPhone 3G was 8 GB but it could also install 130 apps back then when most apps were < 20 MB in size (due to download restrictions)
Then the camera got better, pictures were taking up more space, but around this time iTunes Match came out and pretty much removed the need for space for music so I was still safe with the 4S and 16GB.
By the time I got the 5S 32 GB was exactly what I needed, apps were getting bigger and the carriers upped the restriction to 100 MB so more apps were heavier (especially during the 4S period when third party apps got "fat" with ad tracking code)
I picked up a 32GB iPad Air which filled up after ~40 apps were installed on it along with my ebooks (roughly 700 books). No movies.
I wanted to get a 32GB 6S but low and behold Apple took that out of my options, so I went with the 64 GB because I knew 16 GB wouldn't cut it.
I bought an iPad Pro and I want it to last so I got the 128 GB model to keep me covered for the future.
I recently got the iPhone 7, but I'm afraid to go to 32 GB since a lot of apps are extremely heavy and since the 64 GB wasn't available I went with the 128 GB.
Typically I have about 20 apps that are always installed, and up to another 40 that may just sit on my phone after I no longer use them. It's a trade-off, how many apps must you have installed at all times and how big are they? Once you've figured that out, how often do you want to have to manage the space available on your phone? If you don't mind doing it weekly, which I'll tell you becomes a PITA, get the 32 GB, if you do mind, or you plan on putting a lot of data on it, get the mid-range model.