The measurement "00" in jeans sizes can mean different things based on the manufacturer. Generally, 00 is the smallest size available, but that size can vary depending on companies and styles.
For example, a 00 in American Eagle jeans typically corresponds to a 23 inch waist and a 0 long inseam (32 inches). In comparison, 00 in Levi's typically corresponds to a 24 inch waist and a 28 inch inseam.
As such, it's important to recognize that the size 00 can vary across different brands and styles, especially because the measurements can come in different lengths and waist sizes. If you're looking to purchase a pair of jeans and the label states size 00, it's best to reference a size chart or look up the measurements for that specific pair of jeans on the manufacturer's site.