To be completely exact, using the ruler in-game to measure the sensor's AOE box, it's 5 blocks wide by 10 blocks in height. I found this out fooling around with a player sensor teleporter set up. The box prevents it from working because the sensor, and by extension, the teleporter, activates the moment you're in range. They're not the best sort of thing for this because of that.If it helps at all to have more exact distances:
I was messing around with the sensor and it looks like there can be 10 blocks of vertical space or just under 2 blocks of horizontal space between you and the sensor and it will still go off.
For doors, it would probably be best to go back to player activated pressure switches wired to open and shut the door when run across. Not as fancy as the sensors, sure, but simpler and less likely to get triggered at the wrong time.