Bigger is better. It depend on what your trying to cover though. If you trying to cover a entire valley I'd make them really big 1024 X 1024 or even bigger. What ever the size of the terrain is the size you should make the texture. When your in the terrain editor look at the size your going to creat it at. 1 to 1 is best. Big stuff big textures small stuff small textures. You don't want the dreaded pixels showing up cause the texture is too small. Don't make terrains bigger than nessessary cause they can become poly pigs.