As Stephen already said in his answer, if you use a modern secure encryption scheme, then you do not have to worry about the confidentiality of the messages.
However, as you say your application is a chat the following part of your question
if I add some extra bytes to the message
seems to be a valid (although maybe "paranoid") issue. I interpret this as follows: Besides being save with respect to confidentiality when using a secure modern encryption scheme, i.e., an attacker will not learn the content of your message,
the attacker, without any other mesaures, can still learn the length of a chat message (up to some minor padding if you use some mode of operation that requires padding).
If you do not want to learn the attacker this information then you could padd all messages to some constant fixed size before encryption. Then, if the choice of this value is appropriate, an attacker could not distinguish for instance whether $A$ and $B$ have a "nonsense" communication like this:
- $A$: hi!
- $B$: waz up?
- $A$: ;)
- $B$: LOL
or $A$ and $B$ are discussing something really intensive, which might be a highly intellectual political debate and suspicious. Clearly, $A$ and $B$ could only send single word message to make it look like such a conversation, but the number of sent messages may reveal that this is not the case.
Another thing that comes up to my mind is to introduce dummy messages sent between $A$ and $B$ to "obfuscate" the real conversation and to not reveal too much information about the real conversation.
I do not want to make any statement about which measure makes sense or how to choose this fixed message size and even if you do so you might still leak enough information as a "nonsense" discussion might only involve very short messages and the said highly intellectual discussion on a political topic might involve very long messages. Thus, choosing the fixed message size to even hide such an information might make the chat application too bandwidth and cost intensive (from a computational point of view). Furthermore, dummy traffic clearly also introduces additional costs.