Complex sentence: At above sea level, it is by far the highest park in the majors (Chase Field in Phoenix is next at), and designers knew that the low air density at such a high elevation would result in balls traveling farther than in other parks.
Split into simpler sentences: At above sea level, it is by far the highest park in the majors. Designers knew that the low air density at such a high elevation would result in balls traveling farther than in other parks.