We recommend always talking to a professional about your piercing and its specific sizing needs as everyones anatomy is different. Feel free to ask your piercer when getting the piercing to let you know what was used so if you decide to purchase jewelry at a later date you know what you need to be shopping for (please keep in mind however, most times piercers will pierce with a longer piece of jewelry to accommodate for swelling - which is why coming back for your resizing appointment is so important)
The diameter or lengths are harder to estimate or refer to as common, simply because it’s dependent more on your anatomy. A piercer will always do their best to choose the right gauge, length and type of jewelry. Please remember that there is not a very easily defined right or wrong with piercing or tattooing, the artist just tries to make the best educated choice and unfortunately that allows for human error. Please don’t walk into another shop to ask an opinion and take their piercers response as correct, this industry has a terrible habit of trying to undermine each other and trash each other based more on wanting to steal business and hurt their reputation rather than actually helping you by giving you an honest opinion based.
If you have an issue with something please, first speak to the original piercer, if you are unsatisfied with their answers or solution then go to another reputable shop and ask for their opinion.