Finding out the proper ring size for your loved one is simply one of the most important things your can do in preparing to purchase a ring. For the most part, it is not advisable to purchase a gift ring in a standard ring size which you plan to have adjusted later. The degree to which you can size a ring up or down is greatly dependant upon the design of the ring, and it is almost always better to simply have a ring made up in the proper size rather than trying to size a stock version.
For simple ring styles like a plain diamond solitaire, it is fairly simple to perform size adjustments as much as 2-3 full ring sizes up or down, and even more. However, if a ring design incorporates accent stones or detail work down the side of the band (shank), your ability to safely size the ring begins to disappear. At the extreme end of this spectrum are eternity rings, which have stones set all the way around the band. These ring designs are simply not sizable at all.
To size a ring, a jeweler must make a cut at the bottom of the ring. If the ring is being sized up, the metal will be bent outward, and an additional portion of metal will be soldered in to close the gap. If the ring is being sized down, the jeweler will cut out a portion of metal at the bottom, and the sides of the band will then be bent in and soldered together to close the gap. In either case, this type of adjustment can result in structural damage to stone mountings. This can lead to cracked stones, destabilized mountings, and a variety of aesthetic problems, the most common being an “oval” shaped ring. Ring sizing is also problematic if you have engraving or filigree detail, as the size adjustment may make it difficult or impossible to mimic the original detail design.
However, all this being said, small size adjustments of ¼ size or ½ size are generally not a problem. Fortunately, the average individual will not go through large ring size changes in their adult life. Normal weight gain or loss will only result in small ring size changes which should be easily accommodated by a skilled jeweler. My main intent here would be to caution gift givers who don’t know their loved one’s proper ring size. The majority of ladies rings are made up in a stock size of 6 to 6 ½, but it is normal for women to have ring sizes as small as 3 and as big as 9 or larger.