Why modern ASW rockets are usually 300 mm or larger

What is the smallest viable caliber for a modern surface-launched ASW rocket?

It depends on fuel and warhead, but most practical modern systems are about 300 mm or larger because they must carry a useful depth-charge payload.

There is no universal minimum caliber because the answer depends on propellant, range, and how the weapon delivers its payload. The controlling requirement is usually the size of the depth charge or other anti-submarine warhead. Smaller experimental approaches have existed, including work around 130 mm, but most modern systems are 300 mm or larger. The Turkish Roketsan system is a contemporary example worth examining. Caliber follows the underwater effect required rather than an arbitrary gun-like standard.