I have already designed the powersupply using the Top Switch TOP234Y for +12 and -12v in a single transformer.

The web site of power intergeration gives the complete software for find the no. of turns, core size, and wire size.

The Pcb size is 2.5 inches by 4 inches.

The powersupply works on the flyback mode.
It has inbuild High low voltage cutout for primary, and also inbuilt primary current limit.

The only problem will be winding. The start and the end turns are important for the proper working of of psu.

Need any help email me.

Regards..kannan