What is the recommended size for a height map (terrain) to import in Unreal Engine

New to Enreal Engine and would like to know what size is commonly used in games, what is too low and what is too high. What is the safest resolution
4000x4000, 4096x4096, 6kx6k, 8kx8k, 16kx16k?

It depends on your size, best is to assume that 1 pixel in your heightmap should represent 1 square meter.