The quick brown fox jumps over the lazy dog
It should be 513x513.
–Eric
Could be a number of things. As Eric said, should be 513 x 513 but 512 x 512 still works, you just got an edge of wild pixels. 
Now, you need to also make sure your terrain height is set high enough… if there isn’t much variance in your heightmap you won’t see any change in your terrain.
The quick brown fox jumps over the lazy dog