Grooms are always in trouble regarding this question. Our advice is the following: It’s a good idea to arrive with a ring worn by the lady, so we can gain the required knowledge via measuring it.
If there is none provided, we often start from scratch having the figure of the bride in mind, as it can give a slight impression of what sized ring she might need. Naturally, there is a chance that our speculations are wrong, but our experiences tell that this approach works out fine in use.
However we may carry out the order, it’s always the „premiere” when it comes to light whether the ring fits, or not.
Because of this, we adjust the size of gift rings for free for our clients, of course in the presence of the bride.