As already pointed out there are too many variables. Male / female have different growth rates, genetics some animals are longer and finer than other whom might be thought of as short and stocky. I know not very helpful. Like dogs you should be able to press on the side a feel ribs (gently) but the body should be rounded and not have an angular look and the spine should never form a point. It should be surrounded by large muscles (spinal and semi spinal muscles) and not be protruding. This is not the case for all snakes some arboreal snakes have a diamond shape and more pronounced spinal structure to support more mass farther.