When choosing the size of diamond side stones, the main things to consider are the size of the centre gemstone and the number of accent stones you have chosen. These two factors allow achieving a balanced final design.
As a general rule, the centre gem is the biggest, hence the most visible. The side stones serve to complement and emphasize it, meaning they should not be too big to take the attention away from the centre stone. At the same time, they should not be too small, otherwise, the overall design will have a disproportional look.
In case you have chosen to have more than two side stones, it is recommended to choose diamonds significantly smaller than the centre gem. If you have chosen two side stones (a three-stone ring), they may be of the size as the centre diamond, slightly smaller or half size.
Another aspect to consider is the dimensions of side stones. Accent stones should have very similar measurements to appear in proportion to each other. Make sure their size does not vary by more than 0.2 mm in length, width and depth.