for relocating the cat under the car on a dx, how much length will i need, i found a site that sells plug and play extensions, they come in 12, 24 and 36 inch

 

measure how far you are moving the cat. then decide how much length you need.

you might want to consider i you are moving the cat too far back then that increases the risk of the wires landing on the exhaust and melting and causing the sensor to short out and u may need a new sensor