The length is more important then the width. The water is pushing down on the bubble of CO2. If you give too wide of a tube the CO2 will just reroute around the incoming water flow. The length give more dwell time and will reduce your chances of bubbles coming out of the output into your tank. I used 1.5" but my reactors almost 30" long. This is hooked up to the output of a XP3. You should be good with 2", but I wouldn't go any shorter then 24" total length.