What are the steps to follow when sizing a main circuit breaker?

You have to follow local codes for sizing equipment. In the US it's the National Electrical Code. You'll need to make sure you know where these demand load values came from and how they were determined. In general, the NEC requires sizing based on connected load after certain factors are applied in some cases. I'm going to guess you will need more than a 600 A breaker and switchboard if the actual downstream loads total 600 A. If you're not in the US, you'll need to follow whatever local codes apply.

That is covered in the Canadian Electrical Code; Section 8 — Circuit loading and demand factors, and other sections.
It is unusual for demand calculations to come out to multiples of 100 Amps.
When the calculations for four panels all come out to even multiples of 100 Amps, that may be unusual⁴.
In some instances, the main breaker maximum size may be limited by the busbar ampacity.
However,
In a case where one or more large motors start simultaneously, you may find that a breaker larger than the switchboard ampacity is allowed.
If your sub panel demands are actually calculated and not a WAG based on the main breaker size in the panels, then your switchboard may be overloaded.

Bottom line: You can't supply 700 Amps of demand through a 600 Amp breaker.
But,
There may be exceptions for motor loads, and,
I doubt that your figures are the actual demands of those panels.
Time to buy a code book.

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Ohm's law
Not just a good idea;
It's the LAW!

Mr james64 (Electrical)(OP)5 Apr 23 22:52
" #1. ....I have a switchboard with 347/600V c/w 600A which has panel A with demand load 100A , panel B with demand load 200A, panel C with demand load 200A, panel D .....#2. I am sure if 600A is right main breaker size or not. Can you please advise how to size main breaker of this switchboard ? "
1. 347/600V is rather unusual, could it be 400/690V 50 Hz, or.....60Hz ?
2. If your source is 60Hz, refer to NEC and local codes.
2.1 If your source is 50Hz, refer to IEC 61439-x , BS7671 and local codes/standards ....
Che Kuan Yau (Singapore)

347/600 is probably Canada, thus Bill's reference to the CEC.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

Thank everyone for all comment.

To waross;

All panels were example panel, not real multiple 100A. I just want konw how to size step by step.

If I assumed all loads should be demand load without motor load, should I just add 100+200+200+200 or can I apply for other factor for main breaker ?

700A > 600A so the main doesn't seem rather poorly sized.

Read the code!
The answer to your question is given in Section 8.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!