Depends what it's for, and what scale you're working to. I use 1 unit for 1 metre, and because mine is an FPS I try not to go over 5-10 metres per square. But for a flight sim maybe 100m is fine. And the detail again depends - how big is the heightmap, and how many triangles can you get away with before you get some slowdown.
You could always give the player the option to scale the detail, as well.
You could always give the player the option to scale the detail, as well.