If you're unsure of your ring size, can you purchase a ring size gauge.
I have ring sizers available to purchase.
The best time of day to measure your finger is at the end of the day, because that is when they are at their biggest. Your fingers are at their smallest when they are cold so avoid measuring at that time.
Rings which have a wide band will need to be half a size larger that your normal rings so consider this when checking your size.