It makes a difference for a really gassy dog. Smaller bits are easier to digest. I'm sure the same goes in general too, thats just wht our vet told us when Derringer was having a lot of problems with gas.
Boxers in general tend to eat fast because of the way their jaw is shaped - reason why royal canine boxer formula is specially shaped in the form of an S...to encourage them to slow down. Therefor the smaller the kibble the less they are going to chew it (or in some cases even chew it at all) which can lead to all sorts of GI upsets. Puppy food though is meant to be small so its not as hard on their teeth or their baby stomachs...that said there is also "mini" puppy food for small breed dogs that is even small but im sure thats not what your feeding.
If it is puppy food then it can be smaller food because they dont have enough teethe to support the chewing plus most puppy food is more gentler on the stomach. However, adult food I like bigger kibble like RC because as the term used for boxers and other dogs with similar jaw structure is 'pushed in faces' becasue they don't tend to chew their food. Therefore why smack mentions the 's' shape of RC Boxer Food. They conducted research to see how they could slow down a boxer eating habits and came up with the 's" shape tha made them chew more which caused less gas and less bloat and stomach upset. These three are very common in the breed.
I agree with the above on the eating slower....but, beyond the size of the kibble is finding a food your dog eats well without problem and a food they do well on without bad stool, vomiting gaining and maintaining weight...That may be a smaller kibble or a larger one since all foods are a little different. If yours does well on Diamond that is the important thing and the size of the kibble is secondary.
We have used several different brands and the kibble size has been different in most of them. Many are square and very hard. The one we use now is like round tubes (like duck feed). It is very easy to break up and disolves quickly. Like Sharon said the most important thing is how your dog does on the food.