Deb: I take it that this question is directed more toward gay men, but as a girl who has encountered that same problem, I can tell you that there are ways to get around that. I've encountered a couple of guys that were too large (as in fat girth) for me to get my lips fully around their cocks. I've found that the best way to overcome this is to use your lips in a different manner. Instead of focusing on getting the shaft into your mouth, try simply curling your lips around the side of the shaft, working lengthwise back and forth from head to midpoint. With lots of saliva for lubrication, this can be very stimulating for a guy. You can also cup the palm of your hand around the opposite side of the shaft which, along with your lips, will form a sort of glove around his cock that he can then thrust through. Of course, this technique tends to favor those with a full set of lips, but in general it can work for most I would think.
Also, remember that the most sensitive part of the cock is the head, which in many cases you can wrap your lips around without any problem, even if you can just encompass the tip of the head. You can use your hands to stroke over the rest of the shaft while rolling or circling your lips around the head. This can produce intense pleasure for him/you.
Hope this helps! :)