It's incredibly important to get the right size for the ring you want, as wooden rings can only be adjusted very slightly. Jewellers often have different ring sizing techniques so it is advisable to get your finger sized at multiple jewellers and also at different times of day as they have a tendency to swell and shrink. When making my rings I use an American style ring mandrel so I've included a conversion chart incase you need it.
*note* Also remember to measure the actual finger your ring will be going on as each one will be a different size even on opposite hands.
Rings can be adjusted up to 1/2 a US size, anything more than this a new ring will have to be made.