bong joint size question

Whats the difference? i have seen 14.5mm, 18.8mm, and 29.2mm. 18.8 is the most commen right? so if I want to have the most bowl ashcathcer options is that what i should go with? much thx

18.8 and buy something like this

http://www.aqualabtechnologies.com/medical-glassware/18-14-mm-low-pro-reducer.html