Hi,
I'm not sure if size matters, although a normalmotiity rate (the rate at which food passes through the gut) may lead to an increase in pressure. I would have thought it important to avoid constipation thus keeping any extra pressure to a minimum. I've been told that my endoscopy shows that I have a longer than usual colon, yet my symptoms clearly show that I have IBS.
I would add that I'm no doctor and my advice may be way off!
I suggest that a thorough medical diagnosis is what you need. Please do not be overly concerned by test results that keep coming back negative - IBS is something that is diagnosed when all other problems are ruled out.
You may wish to read and reseach how the gut works from some of the many books on IBS.
I wish you well with your 'journey' of health discovery.
Baz