Product review (5 stars): great hygrometer
Bought this to use in a guitar case to check humidity now and then.  About the size of a "Sharpie" marker pen, so it's convenient to place in a side spot in the guitar case. Seems to be accurate within the limits of this type of meter.

The function is a little fiddly, I had to reset it a few times. It's touchy about its batteries and does some odd things when the batteries are placed in/taken out/placed back in (hence the need to hit the reset button.) Just kinda odd/flaky behaviour, but not enough to make me want to return it.