ed25519 ssh public key is always 80 characters long? I am creating some ssh keys using ed25519, something like:

$ ssh-keygen -t ed25519
$ ssh-keygen -o -a 10 -t ed25519
$ ssh-keygen -o -a 100 -t ed25519
$ ssh-keygen -o -a 1000 -t ed25519


But I notice that the output of the public key is always the same size (80 characters):

ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIB8Ht8Z3j6yDWPBHQtOp/R9rjWvfMYo3MSA/K6q8D86r
ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAILYAIoV2OKRSh/DcM3TicD/NK/4TdqwwBPbKgFQKmGZ3
ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAICpuH/fqCFLbAConChyVH6rZzSaxlnHSwQk6qvtPsf5E
ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAICip4hl6WjuVHs60PeikVUs0sWE/kPhk2D0rRHWsIuyL


Is there an option/param like when creating RSA keys that may influence the length of the key, or by design will always be 80 (68 removing the ssh-ed25519) characters?
Is there an option/param like when creating RSA keys that may
  influence the length of the key, or by design will always be 80 (68
  removing the ssh-ed25519) characters?


No, there can't be any such option.
First note that only the last 43 characters of your sample public keys are variable. As this is Base64-encoding, they can at most encode $43\cdot 6=258$ bits of information, which is enough to fit the 255-bit $y$-coordinate and 1-bit for the sign of the $x$-coordinate (this is called point compression).

Next note, that EdDSA (the signature scheme in use here), requires a group to work with and Ed25519 is the variant of EdDSA that fixes this group to the points on the Edwards-25519 curve (which operates on a 255-bit field). Now because your group is fixed and your public key is a point of the curve, it can only possibly have a maximal length of 256-bit (or 80 characters in SSH encoding).

Now there are "more secure" curves (eg Ed448-"Goldilocks") available than the one used by Ed25519 which have longer keys, but the signature scheme wouldn't be called Ed25519 anymore...