Your piercer is the best resource — they'll know exactly what needle size was used and what length or diameter will fit best. When in doubt, ask. If you can't reach your piercer, here's how to figure it out at home:

Gauge: Insert a standard earring post through your piercing. If it fits snugly, you're a 20G. If it moves a little, try 18G. If it moves quite a bit, you're likely a 16G.

Length: Insert a standard post and mark where you'd like the jewelry to sit. Remove it and measure that length with a ruler. When in doubt, size up — a little extra length is always better than too little.