Whatever sugar you decide to use in a recipe, it’s vital that it dissolves into the drink.
If it stays as a solid, then it won’t sweeten and therefore balance your drink, no matter how much you add.
This is one of the reasons why sugar syrups tend to be used more often in cocktail recipes than crystalline sugars. Unless there is a specific reason, such as in Muddled cocktails like Caipirinhas, where the texture of the sugar actually helps to cut into the fruit.
Therefore, to ensure that the sugar dissolves into the drink, recipes will usually call for Caster Sugar, also known as Superfine Sugar, Baker’s Sugar, or Bar Sugar.
Caster Sugar’s crystals are smaller than the usual table / granulated sugar, but larger than powdered sugar. This in-between sized crystal is important because it will dissolve into cold liquids, whereas the larger sugar crystals will not. Whilst also adding that abrasive, crystalline quality that helps to cut into ingredients and release oils during the process of muddling.
This being said, if you’re adding sugar to hot drinks, the worry over the size of the crystal is removed, as hot liquids will dissolve any sugar crystal you decide to use.