Transformer wire size

I need some help in choosing the right wire size for a push-pull transformer that has 12v(min of 9 ,max of 18) input and 350v output.
The Ae=76mm^2,and it is working at a switching freq of 100kHz.From the design i understand that the output is working at 200kHz.
175W input->150W output:so there are about 19.44 amperes at the input(worst case)and 0.428 amperes at output.

You must use copper foil for primary. You can't use round wire because of skin effect.

Only for primary?And could you please detaliate about the copper foil.