To monitor the humidity in our kitchen / bathroom, we purchased this hygrometer some time ago. It is a compact, sturdy meter that blends into the environment. Easy to measure the relative humidity, because when we shower or cook, we see the values rise quickly - and vice versa, when ventilating (and in good weather conditions) the value goes back down. It gives us a nice indication to keep the humidity under control a bit.
To indicate how big the meter is, I made a comparison photo with an orange.
The only downside would be that it is quite expensive to be so small. But we gladly gave it because it helps us a lot.