Some years ago a wild fox snake (Pantherophis v. vulpinus) got into our backyard pigeon cages and killed a 2 week old pigeon. The fox snake was approximately 1 inch in diameter, while the body of the pigeon was four or five inches in diameter. The snake got the pigeon's head and neck down, but he was blocked at the body. He was determined to get that bird down, though. I stood and watched for a few minutes while he struggled. But when I picked him up, he spat out what he had been able to swallow.
Most of the time a snake can spit out prey that is too big. There have been times where the an oversize prey or prey with horns or something killed the snake that ate it, but those cases are relatively rare. For what it's worth, I once fed a 30 pound turkey to a 10 foot Burmese python. The snake took several hours to swallow the turkey and then took two weeks to sleep off the meal.