Only the length of the message.
The issue is key management. Both parties need the same key and it has to be at least as large as the data you want to send. Each set of parties needs a different key.
If you had a method to securely transmit such keys then you could just transmit your data over it instead.
This is why one time pads are only used by countries to communicate with staff overseas. You can send the pads by diplomatic courier for use in communication later. There is no equivalent mechanism for your web activity and every site on earth.
That's what happens with passwords.
If I connect to https://www.SomeWebsiteIveNeverVisited.com/, how is the web server supposed to tell me where to get the key? Or if I, the client, am choosing where to get the key, how do I securely tell the server where to get it?
Passwords work because they're being sent over TLS which we've decided is "good enough".
Both parties can deduced that offset from the messages they perform.
Before reaching the end of that key, they can agree on another source for a new key to continue the communication.