To represent a scale size of the planets where one planet's diameter is approximately the distance across the United States, we need to consider the size of the planets relative to their actual diameters.
The average diameter of the United States is about 2,800 miles (approximately 4,500 kilometers).
Among the planets in our solar system, Jupiter is the largest, with an average diameter of about 86,881 miles (approximately 139,822 kilometers). If we consider a scale where Jupiter’s size is represented as the distance across the United States, then using that representation, other planets would scale down accordingly.
However, in terms of just relative comparison, Earth has a diameter of about 7,917 miles (approximately 12,742 kilometers), which is still far less than the distance across the United States.
Given the size of planets, if you were to diagram planets where one of their diameters approximated the distance across the United States, it would most reasonably be Jupiter due to its massive scale.
To sum it up: Jupiter's diameter is the most likely candidate to represent a distance comparable to the width of the United States when using a scale diagram for the planets.