Sai
Sai are a category where even small dimensional differences matter. This group includes rubber training versions, metal models in chrome or black finishes, and a case for carrying a pair. At first glance the changes between 18, 19.5 and 21.5 inch variants may seem minor, but in hand they immediately affect reach, balance point and comfort during forms practice.
The recorded data includes overall lengths of 45.5, 49.5 and 54.5 cm. Monouchi length appears at 30 or 34 cm, while yoku spacing is listed around 4.5 to 6.5 cm. Those are exactly the figures worth comparing against the experience of the person who will train with them. If the equipment is meant for regular classes, it makes more sense to match size with an instructor than to pick the first version that looks good in a photograph.
Pair, case and finish
It is useful to think about the full pair and the transport method at the same time. Rubber training sai serve a different purpose from heavier metal versions, and a case helps keep the set together while reducing knocks during transport.
After practice, wipe the surface with a dry cloth and check whether any looseness has appeared around the grip or yoku. Chrome plating, black coating and rubber each need slightly different care, so cleaning is best matched to the actual material rather than treated as one routine for every version.