Copa or highball, and which size
Size is the most important choice and the most underrated one. A gin and tonic consists of about five centilitres of gin, twice as much tonic and a generous amount of ice, and that calls for at least 50 cl of glass. Go for 65 to 80 cl and you can genuinely pack the glass with ice, which is no excess: a lot of ice melts more slowly than a little, so your drink actually waters down less. Anything smaller than 50 cl forces you to choose between ice and tonic.
The second choice is the shape of the bowl. A classic copa with a slightly tapered rim holds the scent best and is the logical choice if you work with botanicals. A tighter, straighter bowl such as the 53 cl Alkemist looks more modern and suits a minimalist bar better. The lower versions of around 53 cl are more stable on a terrace or a tray. Watch the stem too: a short, thick stem is less likely to break and is the sensible option for the trade, while a long stem looks more elegant and keeps your hand further from the ice.