Most compact method of powering Arduino from wall socket There are a lot of methods to power an Arduino from a large range of voltages:


USB cable from PC or from a phone charger or an USB hub
step down converters
step up converters
switching power supply
batteries (connected to the power jack or USB or to Vin)


What I can't seem to find is a small 220V-5V converter. I'd like to put an Arduino in the wall, under a light switch, so size matters. 

The last resort option is to open a 5V phone charger and replace the metal prongs that go in a socket with two wires, and replace the USB socket with another pair of wires that go the Arduino board, but still, an Arduino Pro Mini or an Arduino Nano is smaller than the phone charger.
I can suggest you to try this : AC DC Step Down Converter

It is as small as the Apple's tiny cube, and it can be placed inside of a wall easily. (I'm personally using this with an arduino micro)