That's exactly what it is about. (And some adjustments of the magnet to apply its field in a good spot on the sensor.)
Nope, size does not matter, its all about the strength of the magnet.
I would go with the 6mm magnet. Leaves you enough space to adjust the position of the sensor right. When I was doing this hack, it took me like half an hour to put the parts in and ssome couple of hours to adjust the sensor, that i use exactly the part of the magnetic field, that gives me the best values and the widest possible range on my axis.
The Voltage should not be the problem. there is a voltage margin in which electronic parts work. a little bit less voltage should normally not be a problem. it would be bad, if it was much more voltage.
Did you also use the A1302?