Is there any input (parameter) into the radar for the ship`s length, which eventually influences the cpa ? in other words is the cpa calculated from the point where the antenna is installed ( plus the ship`s length or without the ship`s length ? ) e.g. for a ship of 300 m, the given cpa is for the bow or the antenna location ?

The CPA is calculated from where the antenna is installed.

There would be no point to adjust that reference point by any length forward, since CPA can be referred to any part to the vessel (stern and sides included) and not just the bow.

There is another measurable element which is called BCR (Bow Crossing Range) which however is mostly useful to determine whether the target is to cross the bow or the stern (if negative). On this case, it is again calculated basis the antenna position.

You may think that if you add the vessel's length to adjust it to show the actual distance from bow, then you should again substract the length in case it is to cross the stern, so there is no actual gain.