Selection of proper wire for Ferrite core

Please help me that how do I determine proper wire size in SMPS for building a ferrite core transformer.

I need more information. What frequency?

A very rough idea is to choose the wire size (cross sectional area in cm^2) by 450A/cm^2.

At high frequencies I try to wind most windings in one layer, maybe two layers.
I try it fill each layer completely.

Thanks for help. The frequency is 50 KHz.
Further a rough idea was provided by you, which I understood as mentioned below:

(cross sectional area of wire in cm^2)(450A)/cm^2.

50khz is not terribly fast so you probably will not have to worry about ‘skin effect’ but you may want to read up on the subject. Large gauge wire should not be used at high frequency.

If you have a low voltage winding with few turns that fit in less than one layer, try to spread out the turns to fill one layer. You can use two wires in parallel to fill the layer more evenly.

In an off lime power supply you can have 800 volts across the primary. The insulation is not good. I wind one layer of wire, one layer of transformer tape, then the next layer of wire. If you scatter wind the primary it is likely that the opposite ends of the primary will cross and stress the insulation.

Keep the primary and secondary separated with 3 layers of tape. You may need to put tubing on the wires entering the bobbin to keep primary and secondary spacing.