When it comes to size, think carefully about how high it might come up against your bed. If you have a floor bed or a low profile frame, don't go for anything higher as this can look awkward, not to mention make things difficult for when you want to reach for a glass of water in the night. Similarly, if you have a high bed like an ottoman storage bed, you might have more space for more drawers and therefore more bedroom storage. A standard nightstand measures around 23"–28".
If you have a low mattress or are looking for something smaller, around 23" is best. Always have your measurements to hand when shopping for a nightstand.