with respect to puncturing associated with channel coding, I have a question.

So, when a code word is punctured at the transmitted how does the receiver know the size of the code word.

This is an example from Wiki - https://en.wikipedia.org/wiki/Puncturing

The original code word is 110110 with code rate of 1/2

After puncturing, the new code word is - 1010 with code rate of 3.4

Second and fourth bits are removed from code word with 1/2 code rate

So, in the above case, how does the receiver know that the code word boundary is 4 bits after puncturing?

That is why any user sets FEC rate at receiver as instructed by transmitter

Ok. So, the receiver needs to be informed about the channel encoding configuration/control?

Yes. A communication system does not need to put all the burden on receiver to detect parameters automatically. Constants such as FEC rate, bandwidth of signal, centre frequency etc can be part of tx/rx protocol. If these are not constants then the receiver have to be designed differently.