I don't think it matters a whole lot what size you do. Just put a finger guard on it so they don't get vaccinated. My DGKs have their choice of a few old ladies I put steering wheel spinners on. They can be converted to treadle or electric later on. My DH made up a finger guard out of electrical wire and a connecter - fits on the back clamp foot.
This is an old Redeye - no motor mount for a hand crank so it has a spinner like Steve said:
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