Best way to power Arduino101/Genuino101 with battery

Hi, I got a Genuino101 (Arduino101) board and i wanted to power it with a battery, i'm creating a wearable prototype and for now i just want to carry my arduino in an arm-band. So i need a battery but i also need the smallest battery i can use with it.

I'm no good with this hardware stuff, so i'm asking you what's the best way to power this version of Arduino with a small battery (a battery that i can fit under the arduino board for example).

Arduino101 specs: https://www.arduino.cc/en/Main/ArduinoBoard101

Thanks to all