Guys,

What is the factor to size the motor as per below starter;

1)VSD
2)Soft Starter
3)star-delta

Example,

For DOL starter, the cable size shall be 2~2.5 from normal rating

Not sure how it is in the UK anymore (been a while since I lived there), but in the US you size motor feeders as follows:

Minimum cable rating = full load amps of the motor (running, not starting) x1.25. I.E oversized 25% from running load.

Circuit breaker sizing = 2-2.5 times the full load amps. This avoids starting tripping out the breaker.
In the UK I think you need to have the circuit breaker rating smaller than the cable rating at all times (so 1.25x in this case) and make it a relaxed trip Type-D (this nomenclature doesn't exist in the US).

There is no need to size the feeder on the starting current under any motor configuration; the starting rush doesn't last long enough to cause heating. Remember the whole aim is to prevent the cables rising beyond their temp limit.

Aside from that above: Check the equipment nameplate, it normally tells you what to use for MCA (minimum feeder size), FLA (running load) and MOCP (maximum circuit breaker size).