I have always used screw wire nut connectors made by KBT. When there was a problem with availability, I bought yellow and red connectors made by another manufacturer. They occurred to be of different sizes. Is there any standard?

Unfortunately, there was and there is no standard. In Soviet time the screw wire nut connectors were even the same color. The only progress that can be traced is the difference in colors that corresponds to the difference in sizes. However, the sizes are not universal for all the manufacturers: so you cannot be sure that the orange connector from one manufacturer will be the same size as the orange connector from another manufacturer. We recommend to use screw wire nut connectors made by KBT, because they are made in accordance with factory technical requirements, they have stability in sizes, good quality and are covered by manufacturers guarantee.