You may have already posted this, but how much does your snake weigh?
I doubt a petco mouse is too big unless you have a tiny hatchling.
The food should be about as wide as the snake is at it's widest point. If you are still unsure, get a scale and weigh both the snake and the food and post the weights here...someone can tell you just from the weights if it's far too big or far too small for the snake.