well as far as I know there are two types of tiles: big, with more variations (often natural things like grasses) with a 2:1 resolution, and small, with less variations (dalaran tiles for example) with a 1:1 resolution. What you could try is to resize the 512x512 tile to 256x256, as it is also possible that it just has a two times bigger resolution but still is a 'small' ground texture.
EDIT: just gave it a shot, and I was right, your 512x512 tile in reality is a 256x256 type ('small') as it has only one variation, so you should be fine once you resized it to 256x256