Which parameter to use to specify the relay in a CAN line communication?
There are several parameters of interest;


Impedance of transmission lines and terminators
Signal currents 
Short cct. currents to  +12, -7V
Internal Switch Ron or RdsOn
Off switch capacitance.


The currents usually follow Ohm's Law if you have an accurate model for Ron and drive levels  for PFET and NFET to short voltage for Hi and Lo.
 

The CANBUS is already a 30 port MUX with stubs on a bus terminated by 120 Ohms.

But you may want to consider all options such as FETs and miniature Reed Relays to MUX to many busses.

Start with good Specs.