Necklace
Most Non-Custom Necklace are made in 45cm + 5cm
For the appropriate Cm to choose depends on you, since it a custom Necklace you can have it as short or as long as you want it.
Here is an image to guide you.
Earrings (Hoops)
Are you wondering how big 50mm or 25mm is, I would love to give you a personal recommendation, everyone preference is different. So here is a Chart to guide you to make your decision