I'm no vet but my first snake had the same problem. It seemed it was already in trouble when I bought it and I was unable to get it to feed. When taken to the vet he was put on a liquid force feed and I think it was iodine on the infection. He later died as he was too weak and I learnt a bit the hard way. I now have another of the same sort and 3 others all doing well.
The vet said mouth rot stops them feeding and the lack of feeding urges on the mouth rot, a cycle which I was unable to break. If you can get your snake to feed by its own accord then do it but as you said only appropriate sizes. If your snake will not feed then get him to a vet asap, do not hang around to see if it feeds next time. As I said I am by no means totally clued up on this so I hope others reply but I do think it is important you get it to feed. Best of luck, hope i helped a bit.