There are many factors that determine your ring size, which can change from day to night and season to season, as follows:
Temperature – Fingers have a smaller diameter in cold weather and a bigger diameter in warm weather
Humidity causes fingers to swell
Diet – Salt and other foods can cause you to retain water and swell
Weight loss or gain
Time of Day – Fingers tend to be a smaller diameter in the morning and bigger at night.
It is advised to have your finger sized in the middle of the day in a room that is room temperature.