Standard ear piercings are usually pierced at 20g or 18g. Gauge sizes go up (or down, depending on how you look at it) in even numbers from there, so the next largest size from an 18g is 16g, then 14g, then 12g, and so on. When you get to 0g, the next size is 00g (pronounced “double zero gauge").

So higher numbers (like 16 gauge) are thinner than smaller numbers (like 6 gauge). References to a "larger gauge" means bigger around, not a larger number. That is to say, if you're told that you need a larger gauge than 14, you need to look at 12 or 10 (which are wider), not a 16.

18g is 1.024 mm and 16g is 1.290mm. not too big of a size difference. you *may* be able to get the 16g jewelry in if you try it, but if it starts hurting too much then don't push it further.

Go to a piercer and get them to measure you. That's the only way to be completely accurate, they have the knowledge, and more importantly correct tools, for measuring that kind of stuff.