Sizing rings is among the most common tasks a jeweler is called on to perform. To protect yourself and the jewelry piece, check for weak points in the metal that may break if stressed. Cast rings may be subject to damage if stretched on ring-stretching equipment—especially where porosity exists.
One alternative for safely enlarging cast rings (particularly if you need to enlarge them more than one size) is to use a roller-type ring enlarger.
When sizing a ring, check to see if there is an existing solder joint already in the shank. Solder Prints joint detector is an easy way to do this. If there is a joint, this will be a natural weak point, and can snap if stretched.