[Simonpie]

The number of windings gives the "oomf" to the motor. The area controls the resistance of the whole length of wire. The challenge is, can all the windings needed fit in the space available? The geometry isn't quite as simple as "what diameter gives me the same area" because of how the wires can nest into each other. Also, some people successfully run a 6V starter or coil on 12V (for a while) so there's got to be a bit of wiggle room to overdrive the thing a bit, either on diameter or windings. Another, smaller, factor is that if the wire gets thicker, each winding layer is farther away from the iron core. This distance reduces effect, like magnets losing force when further apart.
A lot of it depends on whether you're smart enough to stop cranking after 10 seconds and turn your gas on, or of you just crank away for a full minute.

This is all just theory for me. I'm no motor winder. If you get advice backed by experience, take it.