There should be no problem mapping a 1024x1024 image onto a 30m x 30m patch of terrain.
If you are loading the image as a Heightfield, you can add a Heightfield Resize operator to change the scale of the heightfield without destroying any pixels in the heightfield image. The Heightfield Resize operator has two parameters:
Re-sample pixels
Re-size in metres
You want to use the latter.
Alternatively, you can use an Image Map Shader to displace using your image. In that case you choose the size in metres in that shader.
Matt