What is the output hash value size of MD6 at its minimum?

The output hash value size of MD6 at its minimum is 128 bits. MD6 was designed to be a flexible hash function, capable of producing variable-length outputs. The minimum output size for MD6 is specified as 128 bits, which aligns with a common standard in cryptographic hash functions aimed at providing a balance between security and efficiency.

While it is technically possible to have smaller output sizes, such as 1 bit, those wouldn't meet the practical requirements for collision resistance and security that hash functions are intended to uphold. Therefore, the choice specifying 128 bits represents the minimum secure output size for MD6, making it the most appropriate answer in this context.