How can I make extremely huge terrains in Unity? It seems like I can set width and length to large values. But the Heightmap resolution only goes up to 4097 and the Detail resolution only goes up to 4048.
Split the terrain into square "chunks", load those you care about (mostly: Those near the currently active camera) in Update() and - if you are strapped for space (you likely will be), unload the not needed ones far away.
Use pre-calculated low-poly models for far-away terrain LoD, unless you don't mind having a low view distance. Also, if you need the terrain heightmap of far away terrain for anything (like NPC movement or some other simulation), create a low-resolution or variable resolution variant of your terrain to keep in memory; don't use the full terrain data for it. This can be done automatically on game load, but generally it's a better idea to make a tool which creates this pre-generated data during the build or package.
Finally: To avoid floating point precision problems, you'll have to reposition the whole scene every time your main camera moves far away from (0.0f, 0.0f, 0.0f).
Possibly relevant question on Unity Answers http://answers.unity3d.com/questions/17225/dynamic-terrain-loading.html
You'll need to break up the terrains in chunks, ie. several smaller terrains. Yo'll need to 'seam' those agains eachother (with a script) so there won't be 'holes' between them but otherwise you can just start doing them right away.
You might want to know too that since Unity3D ~3.0 you can't move items (read terrains and items) too far away (like +/-100000 steps IIRC) so this will influence the maximum size of your world.
I'm using this in my online RPG game anyway (with the exception that I only use Unity3D for level design, not for the actual game).
Just because I'm curious, would you like to share some info about your game (like advancement, type, graphics, style etc.)?