You have a few options in this case:

1. Tile 'em. Textures in 6.4x can be tiled quite well.
2. Scale your texture to the appropriate size. This can cause a loss in detail, however.
3. Increase the image size to 512x1024, center the new area, and copy the pixels on the far left side to the left of the image and those on the far right side to the far right side of the image. Doing this helps reduce odd artifacts you may see from the extra space on the sides.

Although many modern cards support non-power-of-2 textures, such textures are slower and otherwise pose compatibility problems with older cards and mipmaps may not work or behave properly.

I was faced with the same issue with my 3D game's buildings. Either I use a single 512x512 texture scaled to a tiny 1/5.333 scale or I merge three instances in a 3x3 grid to make a larger 1024x1024 texture scaled to a 1/3.556 scale which is much more closer to my target scale (which is 1/4).