What do you call a fluorescent tube that is 5/8' in diameter?

A fluorescent tube that measures 5/8 inch in diameter is referred to as a T5 tube. The naming convention for fluorescent tubes is based on their diameter; specifically, the "T" stands for "tubular," and the number following it indicates the diameter in eighths of an inch. Therefore, a T5 tube has a diameter of 5/8 inch, while a T8 tube would be 1 inch in diameter (or 8/8 inch). The other choice, T58, does not represent a standard naming for fluorescent tubes in this context; it combines the letter and number in a way that is not consistent with the established naming conventions. The choice "Small" does not provide a suitable or accurate name for this specific size of tube and lacks the precision of standardized nomenclature used in lighting.