The bigger the key, the more secure is the ciphertext. However, public key size and conventional cryptography’s secret key size are totally unrelated. A conventional 128-bit key is equivalent to a 3072-bit public key. Again, the bigger the key, the more secure, but the algorithms used for each type of cryptography are very different and thus this is like comparing apples with oranges. Larger keys will be cryptographically secure for a longer period of time. Current thinking is that 128-bit symmetric keys will be safe indefinitely. It is also believed that 256-bit symmetric keys will be safe indefinitely. This is why the AES includes options for 128 and 256-bit keys.