How large can a terrain be that is generated with a heightmap?

I want to create a terrain using a heightmap. I’ve found one of the entire world that’s AWESOME, but I don’t actually know how to go about importing it into Unity. This is because the guide I’ve found has a rather small island terrain. He used the resolution 512x512 for saving the heightmap, and it gave him quite a small island. I wondered if I could use a resolution like 2048x2048 and it would give me a 4x larger terrain. Any help on this would be greatly appreciated!

P.S. I would like the terrain to be about as large as Chernarus from Arma II OA.

Well, honestly, the smaller you make things in games, the better off you are. Having large objects can(and will) slow down your game play if you go too extravagant. For your question on 2048x2048, the maximum texture size is 1024x1024. If you want it to “look” like you have a large area(but you still want it to look realistic), make your player smaller, but not extremely small.