don't buy a 10" key. Purchase a 8mm allen socket with 3/8" square drive. If necessary use with 1/2" drive reducer and a long breaker bar...but you should be able to get it loose with 3/8" ratchet.

Or...less elegant....use the loop end of a crescent wrench on the allen key for leverage. But this technique is for a steady hand and slippage is easier. Much safer with a allen socket and ratchet. Bracing the crank arm is best for leverage as Robert mentioned. I personally have a primitive wooden step and I place my foot over the pedal onto this step. This is pretty imperative for Campy UT torque for example where the center allen bolt torque is 31 ft-lbs...which takes a lot of leverage and stabilize the crank arm.

Check re-installation torque...by finding the PDF assembly manual on line or contacting FSA.