Choosing the right terrain imageicon size

I've been experimenting with the heightmaps by using the grayscale images.

I have tried 64,128,256,512x512.

Ok, I was wondering which is the "best" for larger environments. I like how 128,256 looks, 512 seems too much.

However, I need to make a huge world for fast air travel, and which is the best for expanding, inflation etc/ with fair performance?

Ok, I notice a huge problem for me. I want my textures to be strickly 512x512. But when i stretch my terrain it may loose some.

And I also want it so that my 3 procedural texture are detailed and not ugly unlike the detail texture you apply.

Also, I cant seem to load .bmp, or .tga texture all that seems to work is jpg, and i hate jpg because its lossy.

How do i fix my issuess? I need a huge terrain map with decent geometry detail.

I also need to keep my textures to scale with the terrain and I want all 3 heights to have high-quality textures without having to rely on the detail texture.

How do I go about this?