Size, shape and stackability
For everyday use, 25 to 35 cl is the practical size. Small enough to top up often so the water stays cool, large enough not to reach for the jug every two minutes. If it is a laid table where wine is served too, a water glass on a short stem is customary, because it then matches the height of the wine bowls.
After that, look at the shape of the base and the rim. Toughened glass has a thickened rim that stands up to daily use and is the standard in the trade. A slightly tapered model fits among the stacking glasses and saves a lot of cupboard space. If you also want to use the same glass for juice, soft drinks or whisky, look at the tumbler glasses, because that is the same shape in a wider range of sizes. At home count on six to twelve glasses, and in the trade on at least twice your number of covers, because this is the glass that has to be refilled most often.