The recommended key size for DKIM is either 1024 or 2048 bits. This key size provides a good balance between security and performance, and it is the most commonly used key size for DKIM. It's important to note that the actual key size used for DKIM may vary depending on the specific requirements of the organization implementing it. Some organizations may use larger key sizes for added security, while others may use smaller key sizes for improved performance. In general, however, a key size of 1024 or 2048 bits is considered to be sufficient for most purposes.