Oral cancer IS NOT uncommon in non-smokers, and it is increasing in under 50 year old non-smokers. A lifelong tobacco habit, even one that terminated several years ago, continues to be a causative agent for a variety of cancers, and other medical problems. At this point you should not beat yourself up about choices you made earlier in our life. The past is behind you and nothing can change what was. Your focus now needs to be on dealing with the cards you are holding and playing the hand that you have the best you can.
Informed choices are the main thing that you can do now. Do not hesitate to question your doctors about everything. It is their job to answer your questions as well as treat you. Your RO may not have been to blame for the question, the surgeon may not have forwarded his records or post-surgical path reports to him. This could have been a timing issue or just sloppy handling of the situation. But clearly, asking you about the size of the tumor removed seems odd. You don't mention what his plans for you are, but what ever, you should understand his direction and have confidence in him as a result of his explanations of your questions. Never hesitate to be your own advocate. It isn't the doctor's life on the line here.
If you were not feeling insecure right now, I would say you were abnormal or lying. All of us are afraid of things from diagnosis on. All you can do is try to keep things in perspective, take it one day at a time, and keep moving forward with the vision of a return to a normal life in your future. That does not mean that any of it will be easy or there will not be days when you don't think you can deal with it all. But those days eventually pass, and you will find the strength, courage, and will to face the next one. Trust in yourself. Trust in the great people on this board to help you when it gets rough.