Many rings can be resized, but it is not safe to assume every ring can be made substantially larger or smaller. Construction matters as much as the number on a ring gauge.
What affects resizeability?
Metal type, width, stone settings, engraving, hallmarks, shoulder design and whether stones continue around the band can all influence the available options. Some alternative materials also need very different approaches from traditional precious metals.
Fit should be discussed, not guessed
Finger size varies with temperature and time of day. A good assessment considers how the ring currently fits and the practical change required rather than simply choosing an arbitrary new size.