It will always be hard to submit guys that are larger than you. I'm in a similar situation as you as far as size goes, and I can tell you that as you continue to train, you will be able to submit larger opponents. Eating right, running, sure that's fine, but what will really help you submit these guys (btw outright competition with your training partners should not be your focus), is coming in as often as possible and training.
That said, you will get your blue belt when your instructor thinks you are ready. There is no set criteria, and I would advise not thinking about it. Focus on learning and improving your technique, and if you stick with it, the blue belt will come when it comes. Typically if your instructor gives stripes, there are 4 stripes before a promotion. But don't focus on that stuff, just stick with it and train hard, ask your instructor questions if you have trouble with something specific, and that is the answer to all your questions.