The most accurate way to get your desired finger size is to go to a local jeweller and get them to size it for you.
If you do not have that as an option, you can print out a chart here that will help determine your finger size. It is very important you print it out at 100% with no scaling for accurate measurements.
Our necklaces come in a variety of lengths depending on the design. The best way to see what length suits you is to use a piece of string the same length as the chain and see how it looks around your neck. The same length of chain can look quite different on different people and genders.