The magnetization of the transformer core is proportional to the current in the coil.  And the current in the coil is proportional to the INTEGRAL of the voltage applied across it.  So lower frequencies at a given voltage mean higher magnetization.

Unfortunately, the core has a limit to how much it can be magnetized.  Magnetizing it to that limit is called "saturation".  At that point further changes to the current don't appreciably affect the magnetic field through the core.  It's like the core isn't there and you have an air-core coil with very much smaller inductance.  The current rises abruptly.

Transformers are designed to operate close to saturation.  So running at a lower frequency (with the rated voltage) will saturate them and cause problems.

Fortunately, permanent-magnet alternators change voltage and frequency in sync.  If you get to a particular percentage of saturation at one frequency, then (neglecting resistance) you'll get to the same percentage of saturation at ALL frequencies.  (Resistance makes the voltage decay at low frequencies.  But if you figure with open-circuit voltages you'll set an upper limit and your transformer selection will be safe.)

So the way you pick your voltage and transformer is to use a transformer that is rated for the alternator's open-circuit voltage if it were spun to produce the transformer's rated frequency.

Now you turn that inside-out:  Figure out your under-load RPM of the turbine.  (Presuming for example you're in the US and can easily get 60 Hz transformers) you can build your alternator to produce 60 Hz and 120 V at that RPM and use a 120V primary transformer.  Or you could build one to produce 120 Hz and 240 V and ALSO use the 120 V transformer.  Or you could get a 240 volt transformer and build your alternator to produce 480 volts at 120 Hz, and so on.