Which size of V-bowl you need
Start with the capacity, because that is where it most often goes wrong. A shaken or stirred cocktail comes to eight to twelve centilitres, and that sits exactly right in a bowl of 15.5 to 21 cl: just below the rim, so you can pick it up without spilling. In a 35 cl glass the same amount looks lost. Large bowls are therefore mainly meant for cocktails with plenty of volume, such as an espresso martini with a foam head or a drink you top up with bubbles.
The second choice is the angle of the wall. A sharp V, like the America'20S from Bormioli Rocco, is the classic silhouette but slops over the rim easily when you walk. A bowl with a slightly curved wall, or a Nick & Nora of 15.5 cl, keeps the contents in better and has become popular in bars for that reason. Finally, look at the stem: thin and long looks more elegant, short and thick survives a busy evening better. For hospitality, the sets of six from Onis and RCR are the practical choice.