I think following info will be helpful for same,
for example:
I/P :230VAC or 290VDC
O/P :24VDC, 3.5A

First you have to decide the topologies.

Fabio:
Fly back Topology, i am looking for.

As commented by Mr. Tanaji Kadam
This design can be done by deciding which controller IC u r going to use.Then frequency can be decided.It can be ok with 50Khz,100khz,150khz switching frequency.Accordingly magnetics will change.

Question: What is criteia to decide actually? Ratings and all i provided earlier.
for example:
I/P :230VAC or 290VDC
O/P :24VDC, 3.5A

Usually design is iterative process. So, get your equations in order for efficiency, power ...... and anything else that matters in design, pick a range (say 50kHz to 150kHz that you have selected) and start calculating and graphing your results within your selected limits. Low frequency usually means lower power loss and larger (physically larger) solution. And, cost is another criteria to watch out for.

There is no ONE answer for any design, and the designer is (usually) the only person who knows what is the best answer among all possible answers.

I usually graph my results and pick the best frequency or inductor or .... whatever else I need to select in a design based on the graphs I have.

Your physical space constraints are an important factor to consider when selecting switching frequency. Higher frequencies allow you to select smaller inductors for your SMPS circuit.

Switching frequency selection is sometimes also dependent of load that is after power supply. If power supply is feeding RF electronics or some other sensitive analog stuff.
In these kind of applications it is good practice to select switching frequency so that fundamental frequency or harmonics are not directly interfering (for example local oscillator frequency is something to look for not to have harmonics of power supply).

Topological concerns will be determined by isolation requirements, input voltage source types, the intended product package, safety and environmental requirements, noise, ripple and cost requirements, output power and voltages and their regulation requirements.

Until the topology is derived that best suits the above circumstances, there is really no point in worrying about frequency... After all of that the root concerns will boil down to switching losses versus conduction and losses.

Choose operating frequency based on constraints;
1) size of storage components (LCs), and transformers
2) EMI (very topology dependent)
3) Efficiency
---- switch AC losses
---- parasitic capacitive coupling
---- skin-depth on conductors
4) Magnetics design, ... e.i. switch too fast and you're only driving leakage inductance.
5) Do you need fixed frequency? try variable, it's fun !

sample data:-
I/P :230VAC or 290VDC
O/P :24VDC, 3.5A
Flyback topology.
HOW TO CALCULATE Switch freq.?

switching device : Power Integration IC TOP250
max. switching freq : 132kHz adjustable with R-C combinations

Switching frequency also determines the upper limit of transient response.
As a rule of thumb, the highest transient response is limited to one sixth of the switching frequency (using agressive compensation) for single phase switching.
If you supply a microprocessor, switching frequency has to be in the range of MHz. If you supply incandescent lamps, transient response doesn't have to be taken into account.
Interleaved multiphasing can reduce the switching frequency requirement by almost the number of phases.

Actually you should have to find the frequency response for ur real circuit by using converter transforfuntion. please select the frequency at which maximum gain occurs (tranforfuntion gain). and then according to that u should have to select the IC which will operate at desired freq. select the suitable switching device for the ur converter. otherwise maloperation of the device leads to failure. and also it should be with stand for the flyback voltage+input voltage+safety factor.