In wireless communication, what is "channel length"?
We model channels typically as something with a memory. A typical model for such a channel is the tapped delay-line model. 

In all the models that understand a channel to have memory, the length is simply the difference between the earliest effect and the latest effect of the channel.

In the tapped delay-line model, the term is effectively identical to delay spread, plus – depending on which book you read – the delay of the earliest impulse response.

I think when you read up on the tapped delay-line model, this will become pretty self-evident.