If you happen to choose a suitable magnet system for the size of blades you want to use then you can sort the whole thing out quite easily. Do a test coil and from that you can decide on the number of turns for your cut in speed. All you can reasonably do is use the thickest wire that will get your number of turns in.

If you chose a huge magnet system then that approach would work even if you had to add a load of resistance in the line to avoid stall, you would have the best possible machine but possibly at a high initial cost.

If you happened to choose a magnet system that is too small then you are in serious trouble. You can still get cut in right and you may even get respectable low wind performance but you may never be able to control it in a higher wind, the performance will be poor and it may burn out or run away mechanically and become noisy and dangerous.

If you must do it by instinct then at least look at disc sizes and magnet size that others have managed to get results with, but even then there is considerable variation in what is possible and something chosen arbitrarily that works quite well in one wind area may not work well in other conditions.

I suppose it would be useful if someone tabulated the quantity of magnet for a given size of prop, that would be a good starting point but I am not aware of such information.

Hydro is much easier, if you get cut in right then the turbine will self adapt to some extent. You still need some idea of the magnets to get a given power but at higher speeds you will likely err on the safe side. You can fiddle the loading with the air gap. Only thing to watch is that hydro rating is 100% duty cycle and wind is far from it. A machine that could be rated at 1kW for wind may fry at 300W hydro duty, so don't design for far too low a speed and compensate by increasing air gap.

Voltage at constant speed is directly related to coil turns so increasing turns increases volts or looked at another way it reduces cut in speed. Increasing air gap is similar to reducing turns as far as volts is concerned.

Amps are not easy to deal with, all that you can guarantee is that thicker wire will carry more amps without burn out. It will not necessarily give you more output, especially with wind, unless you use some form of matching other than direct rectifier to battery. The best match comes when you reach about 50% efficiency at 3 times cut in wind speed. If the alternator resistance is too low you can add to it externally, you can't do anything if the resistance is too high.

I am sure this doesn't help you much, that is why people spend lots of time and effort publishing designs that are known to work. It has taken me a lifetime to be in a position to design these things from scratch with reasonable certainty that they will work first time. If you start from an arbitrary point without using the basis of someone's experience you will learn the hard way, as we all had to do before the internet.  

I have given enough information here at various times to design these things from scratch but unfortunately it is not all in one place and I would be the last person to be able to find it.

One thing that I will do is look at a proposed design and see if it stands a chance of working as long as I have all the details.