tl;dr: 5.3 to 5.4 volts and 4 amps. Ideally something industrial-grade.
Ideally you'd get something that outputs slightly above 5 volts in order to compensate for voltage drop.
For example the wall adapter sold by Xulong with their boards actually outputs 5.3 volts (even though the there is 5 volts written on it). Under full load the voltage drops down to around 5 volts across the board (measured at the USB-A connector).
Now imagine powering the board with "only" 5 volts and put a heavy load to it. Will drop well below 5v and this causes malfunction. Also RK SoCs seem to enjoy slight overvoltage