There is way to many varuables here its proportional to the size of the box and the sub aswell as the desired frq. There is no MAGIC port size for every sub size.

 

DD and many others use the rule of thumb of 16 sqin of port to every cuft of box

 

You want to keep the air speed down to less than .06 if the sub is in the trunk, and less than .05 if it's in the car. It's a function of power and displacement.
The bigger the port, the larger the box needs to be to accommodate it.

 

If your looking to do a trunk box facing forwards you should look at larger size ports, however 16 square inches per cube is a good estimate...the box will probobly be 8 cubes and have 130 square inches of port for 2 12s...likely 4 cubes for one 12 if facing forwards with the port forwards and 5 cubes for a 15. Again that is only if the woofer and port are both forwrards, if you change the direction of the port those will definately change to smaller box and smaller port...not sure why but if the port points forwards it tends to like to be big