Can it be cut?
Yes. The product page has some pointers on it: http://www.adafruit.com/products/625
How big is the inverter circuit?
Bigger than you can fit in a Gameboy, for sure. Plus, it is high-voltage, so I wouldn't place it near the DC PCBs inside the Gameboy - instead, plan on having the inverter be separate, and the control wires be as far away from internal circuitry as possible. The battery-powered EL inverter (http://www.adafruit.com/products/564
) lists the following dimensions: (3.4in x 2.2in x 0.85in).