The 4.5" quaich is a generous size to use as a drinking vessel, but not too big to handle. The Scottish tradition is that you fill it with whisky and you pass it around your group of friends. This is a symbol of unity, friendship, loyalty and trust.
Nowadays, quaichs are often given as 'thank you' gifts, or to commemorate an event. In both these instances, a message if often engraved around the outside of the quaich, or inside the bowl.