Encryption is a mathematical process for encoding and decoding information. You can read the size of the key from the number of bits (40 bits, 56 bits, 128 bits, 256 bits). As for a longer password, there are several combinations for a longer key. 128-bit encryption is a trillion times stronger than 40-bit encryption.
When you create an encrypted session, the strength depends on the capacity of the Web browser, SSL Certificate, Web server, and client operating system.