Final answer:
5.1 Surround Sound is not a vacuum tube size; it refers to a home theater audio system configuration, while 12AX7, 6L6, and 6V6 are all types of vacuum tubes.
Step-by-step explanation:
Among the options provided, the one that is not a standard vacuum tube size is Option 3: 5.1 Surround Sound. While options 1: 12AX7, Option 2: 6L6, and Option 4: 6V6 refer to types of vacuum tubes commonly used in audio equipment and other electronic devices, 5.1 Surround Sound refers to a multi-channel audio configuration for home theater systems and does not correspond to the size of a vacuum tube.