Answer: It is a little bigger than a normal pen.
It really just looks like a wide, fancy pen used by presidents to sign treaties with other countries. From far away you could never tell that it was a weird pen and even from close up you would have to know what you are looking for to see the tiny pin hole where the camera is.
The small led lights that show you if the pen is recording or just in standby mode can only be seen from very close up. So don’t worry about being caught.
The small button on the top of the pen is almost flat and does not look out of place. It could even be mistaken for a strange type of click pen. To use the real pen you just twist the lower part of the pen body.
So to sum this question up, the pen is larger than a normal pen but not by too much. To the untrained eye its just a slightly oversized pen. Once your eyes become trained watch out because you might begin to see spy pens in everyone pocket protectors. People might even think of you as fancy when you use the pen. Here is photo to show the difference.