2 ways:
1 with a mesh terrain (ie not standard blitz terrain entity) - uvmap the mesh so that the texture covers the entire mesh, usually with a planar mapping in the xz plane.
other way:
with a blitz terrain entity - scale the texture an appropriate amount relative to the size of the terrain. So if you have a 256x256 texture map for the heightmap then you either scale the diffuse map for the texture by 256 or 1/256 - it is one of those I can't remember offhand whether you scale it up or down.
1 with a mesh terrain (ie not standard blitz terrain entity) - uvmap the mesh so that the texture covers the entire mesh, usually with a planar mapping in the xz plane.
other way:
with a blitz terrain entity - scale the texture an appropriate amount relative to the size of the terrain. So if you have a 256x256 texture map for the heightmap then you either scale the diffuse map for the texture by 256 or 1/256 - it is one of those I can't remember offhand whether you scale it up or down.