Without seeing it I'm not going to guess.
If it's a good amount underweight the spine will be visible, especially when wrapped around an object. It may look pyramid shaped instead of round while laying at rest.
If it's round with no spine sticking out you're probably fine. I'm not going to guess with the numbers alone because the length plays a roll in the numbers. If I'm 4ft tall and 220 pounds I'm overweight but if I'm 6-6 I'm not.
For the food size it should be approx the same size as the snake's thickest part, or just sightly larger.