Assuming a standard 6V 3W dynamo, the front bulb should be 6V 2.4W and the rear 6V 0.6W. These are the highest ratings you can use. Go higher than this and the bulbs won't get hot enough to light up properly, if they're very oversized they'll fry your dynamo's windings.

Most dynamo lights I've seen use MES screw in bulbs, apart from surmey-archer bullet taillights that have a glass wedge fitting. You can't get those anymore.

Nowadays there's MES LED bulbs that are about a billion times brighter, but you have to be careful to choose one that's suitable for Dynamo operation. A wide voltage range (2-9v approx) and no polarity are critical, even then they'll still flash at low speeds.