100 words: Gravity

Given the constants of Earth’s curvature and our rate of gravity, what is the longest length we can accurately measure as being “level” while on Earth’s surface? Assuming you’re using a standard store-bought level, how long does something have to be to go outside of normally measurable length? At some point, the object being measured has to begin curving for the gravity to concede its levelness, right? So how long is too long? Is anything actually level or are things only “Earth-level”? See what I’m saying? Can we even try to accurately measure “level” or “flat?”