It really depends on how much ripple there is on the power generated by the dynamo which doesn't produce steady DC, but DC with ripple. If it's a three pole motor, the ripple frequency will be six times the rotational frequency and will look like the output of a three phase bridge rectifier but will be a bit more spiky.
I would choose 470µF electrolytic capacitor with a 100nF ceramic capacitor in parallel and increase it, if there are any problems.