There are three elements that determine how a generator performs. They are:

Density of magnetic field.

Rate of change of magnetic flux.

Number of wires in the changing field.

The "Science" of how to calculate these is a major undertaking. In actual practice, engineers make a SWAG and them refine the data for a finished product.

To answer your question, IF you keep all other parameters the same, the NUMBER of turns will will change the voltage at a particular RPM. more turns, more voltage, less turns less voltage.

The voltage depends on the flux per pole of the magnet, rotational speed and number of turns. If you kept the centre hole size the same and wound with very thin wire in theory the volts would be the same. In real life the voltage is a distributed thing and not exactly the sum of all the voltages of all the turns so the coil size does affect volts to some extent.

The loosely wound coil will have a longer wire length and will have more resistance than the tight coil and so in theory the tight coil will give a bit more current for the same voltage. With the correct wire size and number of turns the difference will be very little in practice and as long as the loosely wound coil fits it will be good enough. There are going to be variations between individual winders but unless you are so sloppy in your winding that the coils don't fit or you have to change the air gap then you will see no difference and you would be hard pushed to measure the difference on a careful test.

But from experience may I add:-

Wind all coils from exactly the same jig,

Include a trip counter so that it counts and follows the same set up routine.

After winding the coils weigh them on an accurate (digital) set of kitchen scales (for example) and then note down all weights.

The odd extra coil turn or two will make little or no difference.

this is a Piggott 4', sideways magnets and coils.

i wound my 6 coils.  the 1st one came out good, but i was not aware of how to check the 'size' of it at that time.  so i did the other 5 the same since it looked good.  when i set them in my layout, i noticed they were a little too small (SHOULD HAVE CHECKED THAT W/ THE 1ST ONE!!!).  all are the same size and thickness.

now i am unsure if i should use the original magnet spacing as a guide and leave a little space between coils, or push the coils in a little and then the magnets would be a little closer too.  again, maybe none of this makes a difference, or there is no easy explanation!

i would rather not have to make 6 new coils since i probably would be a little short if i tried to rewind my current ones looser...but, if it will make a big difference, i will do it!

thanks again and for the patience of a rookie.