how to determine a length of a channel? ( given path, delay)
With these parameters, Can I define a length of the channel


There's now $n_{tx} \times n_{rx}$ (or, depending on how you consider the this, $n_{rx}$), with potentially different lengths, not just one channel!

The only thing that makes sense to define as "the channel length" is the maximum length among these many channels.

However:

The fact that you're using a working OFDM already says someone else has found the length of maximum length of the channel, and used that as length of the cyclic prefix.

So, really, nothing to do here. If there was anything to do, your OFDM would either be inefficient (too much CP) or wouldn't work (too short CP).