Using adafruit Flora or Adafruit Trinket + MPU-6050 3-Axis Gyroscope + nRF24L01+ together
Using less components fulfills the requirement of "as small as possible".
So usage nRF24LE1 module + accelerometer input + coin battery (CR2032) can be simplest and power-saving solution.
There are nRF24LE1 modules with size of 15x15mm only.

Important thing, running device from coin battery implies wise power management: going to standby and awaking from interrupt. This is possible with nRF24LE1.

nRF24 itself is not Wi-Fi. It's special RF implementation, considered as low-power.
Wi-Fi module is not low-powered and likely won't work with coin battery.