Check here for a full description, but you will see that the sensor can operate out to 20 feet. The diameter of the sense changes with distance but at less than 5 feet the beam diameter is less than 50mm. Remember that this type is designed to be used with a retroreflective target so any other target material will affect response. There is also a gain control on the side of the sensor that you can adjust if the sensitivity is high. Check this at the Banner website.
Part # Qs18Vn6LV
http://www.bannerengineering.com/pdf/63689.pdf