Terrains are chunked by default. If you use terrain LODs(which requires PRO, i believe) it automatically uses multiples of 8 for chunk sizes.
Multiples of 8 thing applies to chunk size, and terrain size should be multiples of chunksize.

There is 128x128 limit for terrain size, but this only applies to unchunked terrain.

From manual:
Quote:
When LOD is activated, chunk sizes are automatically set to a multiple of 8. If the number of terrain mesh squares is not a multiple of 8 in any direction, unchunked stripes remain. For this reason the number of terrain mesh squares should be a multiple of 8 (i.e. 8*n+1 vertices in x and y direction).


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