so 9cf total? 6 ft first chamber 3cf on the second. if you go with 6" round ports im seeing the lenght to be about 9.37" long if you go by what winisd says.if you go with a 7" it would be 13.6" long. i personally would go with the 6"" areo ports

 

at 9cuft you really should be using 6" vents.