Terrain size and 16 bit heightmaps

Hi,

I've recently started a project along with a friend and in total we will need a terrain size of 8192x8192. To get there, we thought about starting with a 2048x2048 terrain, then once finished we'd like to merge it with 3 more 2048x2048 to achieve a seamless world.

Can Unity3D do that?

Also, what tools do we need to create 16bit heightmaps for use with Unity3D? We do not own Photoshop and as far as we know Gimp 2.84 only works with 8 bit color.

We've read this thread: http://forum.unity3d.com/threads/51894-Importing-Terrains

Which states at some point that a heightmap has to have the size of "power of two+1" which means the heightmap we create needs to be 2049x2049 in size, correct? Unity3D does not support a size of 4097x4097 (tried that, we received an error), correct?

Thanks in advance. Frozen1