You need enough turns on the driving side (primary) so that with the applied volt-seconds on one half cycle you do not exceed the peak flux the core can handle, actually you need to keep it lower than this due to core losses due to the flux swing at applied frequency, Erms = Bpk /( 4.44 F N Ae) where Erms = applied volts as a sine wave, Bpk = peak tesla in the core, = 1.2 for normal transformer steel (to keep losses down to manage-able levels), F = frequency 50 or 60 Hz, N = turns, Ae = area of core in m^2, so 50 x 50mm = 0.0025 m^2. So if you know the others, and you do now, you can calc the min N for your application. The wire size is a whole other passel of working out...