Using a T200A-2 for a 1:1 balun, what is the optimum number of turns
and
best wire size? 200 watts max power.


What sort of circuit is it in?
Between what things is it to be used?
What frequency range?
We are not mind readers!

Without further information try 12 turns of Radio Shack, 18 or
16-gauge, twin, stranded, speaker wire. It might work.