The Nano was chosen for it’s compact size with a USB connection for programming. The interface needs an XLR3 male and female plug which mainly determines the housing size.
The other more compact choice is an Arduino Pro Mini which needs an FTDI USB Breakout module.
Power Supply details
The Neonflex comes with a 12V 2A plugpack which will also supply +9V to the Nano by using 4 x 1N4004 diodes to reduce the 12V.