do it! For creating models you MUST have an reference Object to define the width/Height of any object which you create.
If youre not using an reference Object, it could be that all your models have an other meassurement, so they dont fit nicely together.
regards Sebastian
Tutorials:
[Blender]Terrain creation ENG/GER
[Blender]Low Poly Tree Modeling
[GIMP]Create a Texture for Terrains
CLICK HERE
