Selecting the capacitor will be a trial and error process. For a single-phase, 230-volt induction generator, start with 25 uF + 33 uF x motor kW rating. For a 3-phase, 400-volt IG, start with 7 uF + 2.9 uF x motor kW rating.
Use AC capacitors such as motor-run or power-factor compensation capacitors. Do not use motor-start capacitors, they are designed for occasional, short-time use, not continuous use.
The capacitance will need to be changed if the load changes very much. If the load includes any induction motors, sufficient capacitance must be added to bring the load power factor to near 1.0.
For additional information look at this PDF. You may want to search other references on using induction motors as generators. The terms induction motor and induction generator are the more common English terms even though asynchronous is generally understood.