The Proof Size feature in Zero-Knowledge Proofs (ZKPs) refers to the amount of data generated as a proof when a party wants to demonstrate knowledge of a secret without revealing the secret itself. This feature is crucial as it directly impacts the efficiency of the ZKP system, influencing both storage requirements and the bandwidth needed for transmitting proofs over a network. Smaller proof sizes are generally preferred as they reduce the overhead in both storage and communication, making ZKPs more practical for real-world applications, especially in environments with limited resources or high transaction volumes.