You can change the scale of the terrain (or any mesh) using setScale.
terrain->setScale(vec3df(2.0f, 0.0f, 2.0f));
When you create the terrain, you pass a scale as the 3rd parameter. So calling the line above would result in your terrain being twice as large (stretched) on the X and Z axis as in your snippet. Note that the 0.0f y-value in the scale parameter is going to make your terrain quite flat since it will be used to scale the values supplied in your height map.