size must eqaul to 8

OK for learning, bad for production code. Don't limit the password length, it that makes passwords weak. See also https://xkcd.com/936/. Of course, you may want to prevent short passwords, i.e. require a minimum length (8 chars minimum, more required when computers get faster).

Also, you don't want to hardcode a password, or store it in plain text somewhere. State of the art is to use a salted hash (google it), i.e. generate a short random string (the salt), put original password and salt into a strong hash function, store salt and hash value. To verify an entered password, reuse the salt, put entered password and salt into the same hash function, and verify that the new hash value matches the stored hash value.

Regarding your code, there is no need to split the input into an array. Use length to get the number of characters in a string. Also, you may want to use chomp on your input to remove the trailing newline.

Alexander