Some methods to get an accurate ring measurement
(even if you need to get a size without the person knowing it!):
Visit any local store that sells rings. They always have ring sizers and would be happy to size your finger.
Visit any local store that sells rings and take an existing ring that fits. They will be happy to tell you the size.
Please DO NOT guess! Resizing rings is a costly and time-consuming venture!
If your ring is a little big, please see this website for helpful tips and cost-saving methods of resizing - Ring Sizers