Your vagina was designed to be able to pass a baby's head, shoulders, and pelvis during childbirth, so unless the guy you cheated with is truly well enough hung to put any three porn star studs to shame (I mean, I'm talking big enough to make a Clydesdale horse envious), there's no way in hell he could physically stretch you out enough for your boyfriend to later notice any difference. What I think you're really afraid of (and with good reason) is that you're not a good enough actress to hide your guilt, and he'll notice something different/wrong in your behavior around him.

Need I say that cheating on one's significant other is a REALLY bad idea?