I basically want a pocket computer. Access to the cell network is a useful feature, but one I don't need most of the time, so why be connected to it all the time, with all the privacy concerns that comes with.
Despite the name, it's basically just an iPhone without a cellular radio (though I'm not sure if it's actually removed, or just disabled/neutralized). So you can use it to do anything an iPhone can do, it's just dependent on having access to a WiFi connection if you want to do anything over the network.
That said, sometimes you need/want cellular in a pinch, but don’t want it always on, hence the kill switch.
If you have hobbyist-level electronics assembly skills, a weekend, and a couple of hundred dollars, you can pretty easily assemble such a thing using a Raspberry PI with a touch screen, battery, and any of several commercially available cases (or 3D print your own).
I have one that I put together to use for pentesting. It works quite well.
The easiest way to meet the size constraint is to use the Pi Zero. I wanted more grunt than that, though, so I used a normal Pi and stripped the board of all the connectors except the micro USB to achieve the profile I needed.