"I'm just fine" is an unlikely response but, if spoken evenly with no pauses or stress, it would be taken to mean "I'm absolutely fine". However, if you said "I'm just ... fine", the "just" would be taken to mean "only" or "just about" so it wouldn't be as enthusiastic or optimistic a response.
If you have a question about the English language and would like to ask one of our many English teachers and language experts, please click the button below to let us know: