Kaneshin No. 3A is the standard size at a length of 8″ long. These work well on all sizes of bonsai. Concave cutters are designed to create a clean concave cut. You can use them to cut branches and other things or use them to make the edges of wounds cleaner.
For basic usage and tips on how to care for your bonsai tools please visit this link. Bonsai Tools & How to Care for Them